There have been a lot of Slackware updates lately, and one of them affected slackpkg. I got a bit confused the first time it happened, but I learnt how to get around my problems.
The update stopped in the middle of the slackpkg update, and then things went a bit messy as I had to adjust the three /etc/slackpkg/ config files, especially the mirrors. I got through it okay, but when I updated my other boxes, I did it differently. I would update everything except slackpkg, get all the *.new config files checked and cleaned, machine rebooted, and then I would attend to slackpkg. And pay very close attention to the config file updates. Nothing worse than going from slackware-current back to slackware-13.0.
But after I had them all done without any more problems, I noticed one new problem. Blacklisting wasn't working and I had to manually unselect all my blacklisted packages each time. Research on linuxquestions.org gave me a temporary solution. Edit /usr/libexec/slackpkg/core-functions.sh and find each line where the function checkblacklist was called. Change the line to read "checkblacklist && continue". Once you do that, the blacklist will work again.
I read that a proper fix for this would be coming down the pipeline sometime soon.