Thanks guys!
I thought it would make sense when BLTC == BLTDPT to shape the edges but I again fell for the bchg - byte access on memory
.
Beside that I did not get it to work when I fetched the word into a register, bchg-ed and wrote it back...
Pointing BLTDPT to a scratch area does the trick and that works for me for the beginning.
Thanks for reading and commenting on this topic.