i used the boiled linseed oil a few weeks ago and mine, despite the rushed job i did, still looks black!
however, the first time i did it, i then left it in the hot sun, and it melted, lol, so when i walked past it i rubbed it off
so the second time i used a cheapo silicone spray like back to black over the top, which kind of sealed it on and gave the deep black a nice sheen, theres my 2ps worth, but a far better solution than the others ive tried, just a shame i got sh!t loads on the paint