How about these 178,000 men, of whom about 35,000 died: https://en.wikipedia.org/wiki/United_States_Colored_Troops

I don't think their primary motivation was preservation of the Union, I think it was abolition.

Let's not pretend preservation of the Union was the only major factor, or that abolition was entirely a back-burner concept, it had been building steam and popular support for decades.

Sure, preserving the Union may have been the main concern on a realpolitik level early on, but that doesn't change the fact that abolition was the writing on the wall and had been for decades.