The best black threaded headset for less than $50 is probably the Stronglight A9 that Ben's is offering. This headset is no longer made, but Ben's still lists the headset and the replacement bearing sets.

It's not cartridge bearings -- it's roller bearings -- but it's probably as good a headset as anything less than $100.

ADDED: in general, the A9 headsets were not considered to be prone to "indexing." And, if you did have problems with the bearings, you could pop in a replacement set of bearings/races without difficulty.

Here's a photo, lifted from Ben's, that shows the bearings/races:

There's a British shop, SJS, that lists the A9 as "cartridge" bearings at about 33 GBP.

I've seen some ads for very cheap Stronglight steel headsets -- that's not what you want.