There is this black residue on my 23 year old caliper guide pins. Burnt silicone grease or maybe corrosion? Clean up or just replace?

Photo is local
https://www.brickboard.com/GALLERY/volvopic.htm?js_link=&id=13156

Included in photo : I happen to have one new guide pin which I must have bought 5 to 15 years ago. It's slightly different most notably it's not inserted using a 7mm hex (bought just for my 850 brakes) , I seems to be torx T-30 seems a bit loose. I don't own a T-35 (yet).

The two pins were made with different methods.

I wouldn't worry and just wire brush the old ones and reuse.
Here I'd put it on the bench wire wheel.

It might be worth starting a thread about caliper rebuild kits
Since the caliper release is a function of a healthy seal, I suppose that
a lot of calipers fail because of old seals. Look carefully at the boot
around the caliper piston. Once that fails the caliper goes to hell.
The rebuild kits include new seals and boots.

Bill

I use a copper brush to clean them so they shine. Then I coat them with silicone brake grease and reuse them. The black is just baked on grease and dirt.

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Keeping it running is better than buying new

I’d clean up the old one before going with the local Autozone/Oreilly/Advance offering. The goo is probably just old grease and dirt that acculates at the end where the caliper doesn’t scrape it clean.
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Current rides: 2005 Volvo S80 2.5T, 2003 Volvo V70 2.4NA, 1973 Volvo 1800ES (getting ever closer to road worthiness)