About 18% of Union combatants died. Applying that percentage to the about 200,000 blacks (approximately 10% of the Union total) who served gives us about 35,000 blacks who died. Haven't been able to find a source that breaks black deaths out separately.
BTW, better than 1/3 of Confederate combatants died during the war. More men on both sides were killed by disease than by combat.
I will be the first to admit that I am no expert on this, but I was under the impression that very few of the free blacks who enlisted were given combat roles. First, because a lot of the whites refused to fight next to them, and second because the Confederacy threatened to execute all black prisoners.
Triva note: Presidio9 attended the same high school as Col. Robert Shaw, the Union officer Matthew Broderick portrayed in the film "Glory."