My orange tabby had the same thing and I’ve finally figured out what it was. I knew he had ear mites because his ears were always dirty, and I’ve been giving him ear mite medicine (Gold Medal Pets Ear Mite & Tick Control), which called for 5 drops in each ear, twice a day. Problem is that my cat struggled and so I was not doing a good job at getting medicine in his ear. Apparently enough was getting in to drive the ear mites out of his ear, and they were gathering at the tips of his ears. Over about a 2 month period the black at the tips grew larger and larger, and couldn’t be cleaned off with soap, it looked like mildew. Finally someone on google suggested it was not mildew but instead an accumulation of mites trying to escape the ear so I put some of the medicine on cotton pad and used it to gently clean his entire ear, and sure enough over a couple minutes it all came off. I used 3 cotton pads and even cleaned his face and now he is a very happy cat. I also removed the drip tip from med bottle and instead dip a q-tip into the solution and then gently clean inside his ear, which he lets me and I’ve finally cleared up the mite problem that wouldn’t go away for months. I had to share this for anybody else that has black mildew looking stuff on their cat’s ear tips. I guess the black was from the blood that they were digesting.
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