Just so you know, over 9000 BL_ID's would be 9001+, while actual PLAYERS are 9101+ (The first 100 ID's not being used)
When he sold keys, I'm pretty sure he sold blocks of it to third party distributors (for example:
http://www.gamedujour.com/1140-blockland which sold IDs in the approximately 2000 to the???? range, I don't have any numbers here). This means there could be anywhere between a few hundred to a few thousand IDs (if there are other distributors) that don't have an owner.
Also, IDs that were being shared or were posted on pirating websites have been banned.
The over 9000 margin probably won't be passed until 10 or 11 thousand, but that's just a guess.