They are referring to the reserve coverage for a category. Basics - Blue = "positive coverage" and you can drop at trip into open time or swap/drop a reserve day(s). Black is "negative coverage" and you are cannot just drop a trip into open time or drop a reserve day, and swapping days on reserve may not work.

It's much complicated than just this and I'm sure people be along to point out this or that can or can't be done with black or blue days.