I wouldn't have believed it if I hadn't tried it myself - peanut butter is amazingly effective in restoring exterior black trim!
I have a 1991 black N/A MR2 and over the 20 years we have been together I have used all sorts of black trim detailing products off the shelf to try to retain, and in more recent years restore, the black in the side trim, around the windscreen wipers, around the rear lights, and that (ridiculous design) honeycomb piece in the boot.
A car enthusiast colleague told me they had heard about people using peanut butter to restore black trim - I did a google search and found similar claims so thought I would put it to the test, particularly since some of my trim (mainly around rear lights & in boot) was not responding that well to usual products. I bought generic brand smooth peanut butter, put some on a rag, gave a couple of rubs on the no-longer-black rear trim, and presto! the wiped trim was black again!! Yes, you will get a bit of peanut butter in the cracks around the trim if you aren't careful but this is easy to remove (bit of cardboard, soft brush..) and otherwise no peanut butter residual smell.
I've now done all the rear black trim, including the honeycomb nightmare (used a mix of rag, sponge, toothbrush to effectively clean and then remove residual peanut butter).
I've since washed the car and the trim is still black so that's why I thought it was time to share this discovery with you all.
I also had an idea that peanut oil might be just as effective but less messy, but it isn't. Looks fantastic as a finish after original peanut butter clean but will wash/wear off.
Hope this is useful - or let me know if you have any peanut butter detailing experiences!