Pokemon being BL is a mix of power and usage. Pory-Z was originally OU, then BL, then OU again due to its usage changing. Its power remained constant.
I know what you're saying, but, to clarify, a Pokemon is BL when it's not used enough to be OU but too powerful for UU.
So the best OU teams probably consist of 6 out of those 8 Pokemon on that frequently used list...Would this mean it's best when making an OU team to choose Pokemon exclusively from this list or would there not be enough type coverage to go around or something?
I never suggested to choose your OU team just from those 8 Pokemon!
Look at it this way: if I had to view two matches at random last September, I would have probably seen those 8 Pokemon in those players' teams. Meaning that, out of a maximum of 24 different Pokemon in those 4 teams (probably less, since it's very probable that the teams had a few Pokemon in common), 8 of them were those eight. That's all it means.
Another way to look at it is: if you go in the RMT subforum and look at 4 teams at random posted at around September, those 8 Pokemon are more probable to be in at least one of those 4 teams than not.