No idea what is going on. I have several pairs of M&S jeans, all the same style (Lily slim fit). Love them, super comfy. Black ones, light blue, darker blue. No issue.
I have one pair which I think are light indigo - they are quite dark. Every time I wear them, I get itchy skin under the seams of the jeans. The jeans have been washed no differently to other jeans, and all other colours are just fine. Thinking i'm going to have to get rid which would be a real shame as I like the colour.