PLEASE help answer this question for me about my two kittens!!!!?

My 9 month old and 6 month old kittens get dark buildup in their ears every few weeks or so. Its black and sometimes the consistency of ear wax and sometimes more flaky and hard. I try to clean it out as best i can with a q tip. I was wondering if anyone knew if this was normal or something i should be worries about??? I can take them to the vet if i need to. If you have any idea what is causing this please let me know.