Megumi's right. Slaking has a Base Stat Total higher than 600, I believe, but he's a BL pokemon because his ability holds him back. Celebi is a 600 BST Pokemon, but it's OU because it has a trillion weaknesses. Shaymin is 600, but its movepool sucks. It's BL.
I could go on, but you get my point.
While BST does sometimes come up while making tier lists, it's ultimately a Pokemon's effectiveness in regular battles that determine whether it's Uber or not. In the early stages of D/P, Manaphy was a debated uber, but was usually allowed in OU play. However it was discovered that with Hydro Resting and Tail Glow sweeping, he was too powerful for OU play and was shoved in ubers.
Did I help out any?