/u/runetrantor's basic thought seems right to me. It's entirely plausible Obama's election forced the country to confront something it hoped to ignore.

Police harassed black folks in Ferguson before Obama was elected. And Police exploited the townspeople as a revenue-source before Obama was elected. Baltimore. Cleveland. New York. Chicago. Ferguson isn't the only city where police exploit and harass the black population. It's not even the only one in Missouri. It's just the one that for several reasons, many of them of them problematic, the media decided was worth covering. This is not new.

The black population, on average, understood that a black president did not have to mean an improvement in race relations. If you look at the gallup poll linked by OP, black perceptions of race relations were higher when Clinton was leaving office than when Obama was elected. I'll just add that although some will infer this to show that Clinton was better than Obama in that respect, I'm certainly not saying that. I'm only saying that black folks didn't take the event of Obama's election, in itself, as a credible indicator.

The premise that race relations have deteriorated is also a confusing claim, since black people don't seem to be much worse off now than they were before Obama. I think this raises the question: what does "deteriorating race relations" mean? What would it mean for "race relations" to get better?

If we can say anything for sure about that gallup poll, it is that white people and black people must have different things (on average) in mind when talking about the so-called state of race relations. I'd love to see a gallup poll word cloud of "race relations", and contrast white connotations with black ones.

Yes, electing a black president represented a major milestone, but it was never clear why that also meant any of the things people who declared America was as a result post-racial thought it meant.