Buying a bolt is what I do. I keep a bolt on hand at all times. Some "blacks" are dyed to be a true black vs. the printer dumping all colors in the vat to make make a "black". Hence, the variations in black. I also keep a bolt of white in my sewing room. Not all whites are equal.
Michael Miller is a true black as is Blank Quiltings' Eclipse black. I am currently using a double-dyed black from Maywood. As long as you are consistent in using only one "black" in your project you should be good to go.
sandy