Told OP it was a bad idea, was ignored by OP in the Blackcoin thread. If you don't understand the code you're modifying, don't muck with it.

https://bitcointalk.org/index.php?topic=509928.msg5651789#msg5651789
https://bitcointalk.org/index.php?topic=469640.msg5651892#msg5651892

There is another issue with using stake modifiers derived only from PoS blocks as well.