Yes, it as a bug, the problem is a problematic options: 'Vista-ize me' and 'Only blur the background if the skin has blur enabled'.
WindowBlinds 6.09g[b].135 = buggy as hell:
- Vista-ize me > Off (Reset default)
- Only blur the background if the skin has blur enabled > On
Result:

- Vista-ize me > Off (Reset default)
- Never blur the background even if the skin has blur enabled > On
Result (no blur/border shadow problem):

Same problem with 2 computers under Windows Vista.