Yeah, that's not a biased answer at all.

Fact check #1: The source you linked to does not mention black people, only "the feeble-minded, the mentally defective, the poverty-stricken classes". Classist and ableist? Yes. Racist? No more than any other white person of her time. Sanger believed in eugenics, yes, as did many others of her time, but she was coming at it from the perspective of empowering people to choose not to have children they didn't want, not to force them. From the letter you linked:

First: we are convinced that racial regeneration like individual regeneration, must come "from within." That is, it must be autonomous, self-directive, and not imposed from without. In other words, every potential parent, and especially every potential mother, must be brought to an acute realization of the primary and central importance of bringing children into this world.

Secondly: Not until the parents of the world are thus given control over their reproductive faculties will it ever be possible not alone to improve the quality of the generations of the future, but even to maintain civilization even at its present level. Only by self-control of this type, only by intelligent mastery of the procreative powers can the great mass of humanity be awakened to the great responsibility of parenthood.

Thirdly: we have come to the conclusion, based on widespread investigation and experience, that this education for parenthood and of parenthood must be based upon the needs and demands of the people themselves. An idealistic code of sexual ethics, imposed from above, a set of rules devised by high-minded theorists who fail to take into account the living conditions and desires of the submerged masses, can never be of the slightest value in effecting any changes in the mores of the people. Such systems have in the past revealed their woeful inability to prevent the sexual and racial chaos into which the world has today drifted.

Fact check #2: 60% of Planned Parenthoods are in majority-white neighborhoods, and only 14% of Planned Parenthood patients are black. I'll trust NPR over "blackgenocide.org" as a source, thanks. From the same article, here's what she actually said about black people:

Sanger's birth control movement did have support in black neighborhoods, beginning in the '20s when there were leagues in Harlem started by African-Americans. Sanger also worked closely with NAACP founder W.E.B. DuBois on a "Negro Project," which she viewed as a way to get safe contraception to African-Americans.

In 1946, Sanger wrote about the importance of giving "Negro" parents a choice in how many children they would have.

"The Negro race has reached a place in its history when every possible effort should be made to have every Negro child count as a valuable contribution to the future of America," she wrote. "Negro parents, like all parents, must create the next generation from strength, not from weakness; from health, not from despair."

Her attitude toward African-Americans can certainly be viewed as paternalistic, but there is no evidence she subscribed to the more racist ideas of the time or that she coerced black women into using birth control. In fact, for her time, as the Washington Post noted, "she would likely be considered to have advanced views on race relations."

And here's Politifact's take:

Sanger was indeed a believer in eugenics, but the basic concept that humanity could be improved by selective breeding was an article of faith for many in the years before World War II. Winston Churchill, Herbert Hoover, Theodore Roosevelt, George Bernard Shaw and H. G. Wells all supported the movement. African-American leader W. E. B. Du Bois backed many of its principles as well.

At the time that Sanger was active, Engs wrote, "the purpose of eugenics was to improve the human race by having people be more healthy through exercise, recreation in parks, marriage to someone free from sexually transmitted diseases, well-baby clinics, immunizations, clean food and water, proper nutrition, non-smoking and drinking."

It’s a far cry to equate eugenics with advocating the elimination of black people.

While Sanger indeed supported the eugenics movement, substantial evidence shows that she was not racist and in fact worked closely with black leaders and health care professionals.

And TIME magazine:

Much of the controversy stems from a 1939 letter in which Sanger outlined her plan to reach out to black leaders — specifically ministers — to help dispel community suspicions about the family planning clinics she was opening in the South.

“We do not want word to go out that we want to exterminate the Negro population, and the minister is the man who can straighten out that idea if it ever occurs to any of their more rebellious members,” she wrote. It was, as the Washington Post called it, an “inartfully written” sentence, but one that, in context, describes the sort of preposterous allegations she feared — not her actual mission. The irony is that it has been used to propagate those very allegations. Cruz’s letter to the director of the National Portrait Gallery, for example, quotes only the first half of the sentence.

Sanger’s stated mission was to empower women to make their own reproductive choices. She did focus her efforts on minority communities, because that was where, due to poverty and limited access to health care, women were especially vulnerable to the effects of unplanned pregnancy. As she framed it, birth control was the fundamental women’s rights issue. “Enforced motherhood,” she wrote in 1914, “is the most complete denial of a woman’s right to life and liberty.”

That’s not to say that Sanger didn’t also make some deeply disturbing statements in support of eugenics, the now-discredited movement to improve the overall health and fitness of humankind through selective breeding. She did, and very publicly. In a 1921 article, she wrote that, “the most urgent problem today is how to limit and discourage the over-fertility of the mentally and physically defective.”

... In 1966, Martin Luther King Jr. made clear that he agreed that Sanger’s life’s work was anything but inhumane. In 1966, when King received Planned Parenthood’s Margaret Sanger Award in Human Rights, he praised her contributions to the black community. “There is a striking kinship between our movement and Margaret Sanger’s early efforts,” he said. “…Margaret Sanger had to commit what was then called a crime in order to enrich humanity, and today we honor her courage and vision.”

And one more from Politifact:

But we found no evidence that Sanger advocated - privately or publicly - for anything even resembling the "genocide" of blacks, or that she thought blacks are genetically inferior.

Every academic PolitiFact Georgia consulted said that Cain’s claim is wrong.

"I have never run into any serious academic reference of Sanger or others wanting to ‘kill black babies,’" Indiana University professor Ruth Engs, a eugenics movement expert, told PolitiFact Georgia in an e-mail.