Normally "genocide" is considered bad because it results in living people being killed. I think we can all agree that killing a living person against their will is bad.
But with what you describe as "genocide," not a single person is harmed. To me, an African American person has the same intrinsic value as a white person or anyone else. Therefore, I can't see any particular reason why I should care if we have more or fewer African Americans in the world.
I'm familiar with the argument made by deaf people. But those advocates never seem to address the reversal argument; why shouldn't we also produce new deaf people (e.g. by sedating some children and painlessly puncturing their eardrums) in pursuit of this goal?
Skin color is a relatively minor component of race, race describes the clustering of a whole host of traits and characteristics in a group of individuals.
Yellow-skinned Africans would still be Africans, purple-skinned Asians would still be Asians, and green-skinned Europeans would still be Europeans.
Skin color is a minor component of the actual makeup of race - but most of what "race" is, is political, economic and cultural. And skin color is a major factor in that regard, because it's an obvious way to discriminate between in and out groups.
When Obama was first running, there were discussions about whether or not he was "black enough" to really represent African American culture as President. Part of that was because of his upbringing (because, unfortunately, there is the perception that being educated, well spoken and wealthy is somehow antithetical to "true blackness") but part of it was the lightness of his skin color. It would have likely been easier for some people to accept his "white" background if he'd been physically darker.
You yourself correlated European with "white" in an earlier (probably still flagged) comment, so you know that references to skin color can seem intrinsic to many things to which it doesn't really apply.
Some aspects of cultural, ethnic and religious identity are associated with and expressed by genetic factors like skin color.
For African Americans, just to give an example, the premise of "editing" children to be "whiter" might appear to enable racial genocide.
It doesn't apply only to skin color - some deaf people would consider genetically engineering away deafness an attack on "deaf culture."