If those were my plants, my first thought would be that they are severely undernourished.

Doesn't look like a heat issue, but put a thermometer in there to be sure.

Can't remember if it's lack of P or K. Research it a bit and come back and remind me...lol

Kevin

Thanks, I'll try to hit them lightly with fertilizer. I'm using the same kind of soil as last year, and didn't fertilize until June or something, so it's a bit of a surprise, but worth a shot, for sure.

I have had EXACT the same problem with my peppers. Now they are getting better. I call it "PURPLE" not black.

Anyway, I increased the fertilized and kept the lights farther away and shortened the hours. Now no more purple but some are still yellow. I don't want to push them right now because I don't want them to grow too big, inside. Once planted out, I can take care of them, I know how.

Here is a picture of mine about 5 weeks ago

The dark color is pretty common with pepper seedlings. I think it has to do with the artificial light. I have several that are exhibiting the purple coloring on leaves. I don't think it is signaling any real problems and unless the pepper is of a variety where the dark coloring is natural, I think it will clear up soon after getting outside in the real sun. I dont think any of those peppers pictured above look bad at all, maybe a little leggy, but that is it. I wouldn't be worried.
Bruce

I think you are right , Bruce.
Not all my seedling were affected that way. Bu they were all treated almost the same way. The one in the picture ( a Thai Chili) was the most severe case.
It is possible that different peppers have different light or cold sensitivity.