Agree with OsnobunnieO, normally there is no reason to clean the ears of an indoor kitty.
If you ever try to do something like that anyway, please, please, please, be extremely careful, because if you go in too deep, you can puncture the eardrum.
Since even indoor kitties can get an ear infection, watch for head shaking and kitty holding one ear in a funny way, even flat, the tip pointing to the side. With that kind of thing a vet visit is necessary.
Food allergy can cause dark stuff collecting in the ears, pretty much what you would normally see with ear mites. A cat may not necessarily have an ear infection when that happens, but at that point adjusting the diet becomes absolutely necessary.