Apparently every line of /etc/slackpkg/blacklist is now considered to be a regular expression, which can match *any* part of the package name (and/or version, and/or build number, and/or tag). So, in your example, 'gcc' is a regular expression that blacklists gcc, gcc-g++, gcc-gfortran, etc etc. Also, 'glibc' is a regular expression that blacklists glibc, glibc-i18n, glibc-profile, etc etc. To blacklist the whole of Multilib you could just write
Yes, it will probably cause a small amount of confusion until we all learn the best way to use it!