I have done the little hack that is mentioned above because I was unable to post any comment to my site. Then I got the old error I used to get:
"Warning: Cannot modify header information - headers already sent by..."
So I did what was suggested here
and 'dropped the blacklist db from my wordpress DB and rebuilt it with the installer' - but no joy
I still get the same error. I can't locate the offending lines and they seem to be in the default blacklist.txt file.
At the moment it seems I just have to either turn off wpblacklist and get decimated by comment spam, or turn it on and totally confuse people with freaky errors when they post a comment.
Can anyone help me? - I am at the end of my tether....