I have much more than `build` to block. IDE-specific stuff, smalls scripts I write to help me (and go in root) but don't belong in the repo, etc.

And what if for some reason you accidentally copied a big Linux ISO to that directory by mistake. Without a whitelist, you might accidentally add and commit a 700MB file to your main and not even notice. What a pain when you push later and have to git amend, rebase -i, etc.

Better to block all except whitelist. The only downside is it's less obvious how to do this than allowing all except blacklist to new git users.