Does anyone know why some of my chillis have black marks and are they still edible?

They may have been nibbled by bugs, possibly when they were very small. They should still be edible.

Many thanks Volunteer for answering my question.

If your chillies were green with black marks they could have just been ripening to black before going red.

I have had the same problem for the last two years and i would like an answer to this.
I do not think it is nibbles from bugs because the chilli skin is not broken.
I would like to think that it is green turning to red via black but i have never seen the black turn to red.
Are there anyother suggestions please?

The problem might be due to your growing location i.e. outdoors.
Remember, chillies like a warm/hot environment, with night temperatures warm.
Think of where chillies grow in the wild, India, Pakistan, Bangladesh, Thailand, Sri-lanka, many African nations, Mexico, and South America. Lots of sun and lots of heat.
If you're growing in pots try bringing them indoors at night before the temp. drops.
If the dark/black bits on your chillies are soft and 'squishy', then the chillies are probably starting to rot. If they remain firm they are probably just ripening.
Hope this helps.