There can be merit in preserving separate lines, if you have a reason for it and your blacks and blues come from different lines.
But if you're breeding blues it's not possible to keep blacks and blues (or splashes) separate.
You might enjoy reading this thread on the genetics of breeding blue chickens viewtopic.php?f=10&t=7968728
In the article it mentions that it can be a good thing to keep black birds involved in breeding blue ones as that'll help to maintain a darker blue. I'm not 100% sure about that...I breed blue ducks and I find that it makes no diffference to the strength of the blue whether or not you pair blues with blacks or not. But it may be a bit different in chooks. Certainly most people breeding blue birds would include black birds in their breeding stock (I do) simply because there's no good reason not to.
As noted in the article if you breed black to splash you 100% blue offspring - and if you're keen on blues then that can be a very handy pairing. So don't reject splash offspring either.
Another good thing to keep in mind is that type (shape, size, conformation) is always Number 1 when deciding whether to keep or breed from birds, with colour a long way second.