Isn't bl_id 10000, blockhead10000 now?
No, ID 10000 isn't sold yet. Blockhead10000 is ID 9999.
Your blockhead name is one higher than your ID, so ID 10000 would be Blockhead 10001
No, Blockhead10001 has an ID of 10001 now.
ID 10000 will have whatever name it is sold after.
For example, if the 10120 ID (Blockhead10120) is sold before the auction finalizes, the 10000 ID will take up the name Blockhead10121 and offset the names once again.