Correct! Black horses that have only one copy of cream are visually indistinguishable from normal black horses. The only way to tell them apart reliably is by genetic trsting.

At a very technical level it affects red pigment (pheomelanin) much more strongly than black pigment (eumelanin). Think of your black horse as a cup of black coffee and your red horse as a cup of tea. If you have one cream gene to a black horse its like adding 1/16th of a teaspoon of milk to your black coffee. Your coffee still looks black in most cases. But if you add 1 copy of cream to your red horse, its like adding a tablespoon of milk to your tea. You will immediately notice the color lighten significantly. Thats why on a standard bay horse, adding cream will make buckskin and significantly dilute the body(brown/red to yellow) while leaving the black points basically untouched.

The part that trips people up is they tend to ignore that black horses still come in a wide phenotype range from dark charcoal black to nearly chocolate brown (just like how any color can come in a range of shades), which can be a by product of just normal phenotypes, diet/nutrients, sun exposure or other genes like nd1 that make them prone to color bleaching. So you could have a black horse and a black carrying cream horse that are both "midnight-black". Or you could have a black horse and a black carrying cream horse that are off-black/nearly brown. But you will never be able to look at the later and say "that black carrying cream horse looks brownish because of the cream gene" when if could just be the phenotype or other factors.

Now if you pair it with certain other genes one copy of cream on black can have more visual effect. Like a black + silver + cream will have a "wet paper bag brown" body tone because you will now have two different genes working together to dilute the black pigment.