The steps to deblob an existing Slackware installation would be quite easy if not for the necessity to replace the non-free kernel. While the task of rebuilding the kernel is arguably easier in Slackware than in many other GNU/Linux distributions, it is not for beginners. If you don't feel like learning to build the kernel, feel welcome to use our free repository to obtain binary kernel packages. As a middle ground, you can use our kernel build script to make your own packages from Linux-libre source.
That being said, you can de-blob Slackware in a completely manual fashion by doing this:
kernel-headersas if it was free. It is in fact free, and replacing it manually is a pain.
/etc/slackpkg/blacklist. Make sure package names occupy separate lines, and avoid white space.
We haven't tried this, but we don't know of a reason why it wouldn't work. Make a local copy of the Slackware repository, remove all non-free packages, and then upgrade as usual.
Please be aware that as long as you keep using the official repository, non-free packages may still creep in during updates, unlikely as it is. We see no easy cure for this, short of switching to a free repository.