Many packages have a blacklist.txt reason like "is GPL/PerlArtistic License and not GPL" which indicates that the Arch PKGBUILD says license=(GPL), but the source is actually licensed like:

This program is free software; you can redistribute it and/or modify
it under the terms of either:
 - the GNU General Public License as published by the Free Software Foundation;
 - or the Artistic License.

So here's the thing about that: THE PKGBUILD WAS NOT WRONG. We (or Arch) have the choice to distribute it to users under either the GPL or the Artistic License. By saying license=(GPL) in the PKGBUID, we simply choose to distribute it to users under the GPL, rather than the Artistic License. That's fine.

Other packages purportedly have the reverse problem, where it says license=(GPL Artistic2.0), but it is only actually licensed under one of them. This is a bug in the Arch package. Report the bug upstream on the Arch bug tracker, rather than listing it in blacklist.txt and rebuilding it.