steamclient is a x86 package and can be found
here. So it's normal not to find it in the x86_64 repository.
As explained in this
post, blacklist will not work for x86 packages when you have a x86_64 system.
Your x86_64 system is working as designed:
- Search for a x86 package with "slackpkg search" gives no result
- Blacklisting a x86 package will not prevent it from appearing in "slackpkg clean-system"