I clean mine once a month with the chimney brush sweat tent sells. I’ve noticed the blacker it is the more ash you have in your chimney and then little hot ash ambers come out of the chimney when you have a fire going which is no good. I’d just scrub them clean once a month and you Should be fine.

This is normal and happens from ash build up from the wood fire. We'd recommend cleaning your chimney pieces every once in a while but generally this is normal and not a concern!