Alaska doesn't have much arable land, so the "forty acres and a mule" wouldn't work there, and moving African-Americans to what was then a desolate wasteland would not be taken well by the Radical Republicans. If they wanted to get out of "white America", the best place would probably be Liberia, where it would probably be easier/cheaper to take them since they ocean travel is much quicker and easier than land travel.
That being said, the scenario would be interesting. Perhaps the African-Americans would ingratiate themselves with Inuit or even Russian cultures. This might lead to a breakaway of Alaska from the US sometime in the next century, since Alaska is so culturally different than the rest of America.