What the "Merleing Gene" does is make certain parts of the fur lighter (or it makes the topcoat lighter so that the undercoat shows through lighter, I don't rememeber exactly which.) It doesn't effect the whole coat, though, so a blue will always technically have *some* black as well.
A bi-black is just what it sounds like... two colours, black & white. So then the bi-blue is the same, but with the merle gene, so they're blue & white!
A tri-colour is black, white, and tan... and a blue merle is a tri-coloured sheltie but with the merle - in theory they'd turn out blue, white, & tan!
Sables, since you said you probably wanted one, are typically brown, white, & black - but even within the sable, they can vary! Auggie is a blonde sable/apricot sable/red sable or whatever else you might call it... he has very very little black on him. OTOH, my sister's shelties are both "shaded sables" - they have soooo much black on them! Sometimes it can be hard to tell when they're puppies how much black they'll have when they get older. Auggie had a black stripe down his back that we expected to expand and darken up - but, surprise, it actually vanished instead! Er, whoops! He also lost some of the white he had on his face... he had a white lightning-bolt mark on his forehead (we jokingly said he was Harry Potter) that now has turned into a tiny bit of white that you can only see if you get really close and know what to look for! =P
So be prepared when you find a breeder and pick your puppy... your puppy's appearance will likely change when he gets older!
Hope that helps... it definitely can be confusing, LOL! Personally I want a blue merle for my next sheltie. =3