I use milk in black tea quite often. If I'm trying a new to me black tea, I try it first without milk. Sometimes, a black tea requires milk to "smooth" the flavor, and lose a possible bitterness. When I do add milk, in an 8 ounce cup, it would be about 1/2 ounce. Over the years, I have added less and less milk by percentage.
Assams, Russian Caravan tea, keemun, and yunnan all take milk nicely.