Why does my cat have a black blob in his ear after coming back from the vet?

My make cat just came back from the vet. Desexed and microchipped. He seems to be doing fine. But I found this black tattoo like blob on his left ear. What is it? I don’t really know if it’s an tattoo or a marker. But I did read online that vets like to put a M to say to people that his male.. is that what it is??... show more My make cat just came back from the vet. Desexed and microchipped. He seems to be doing fine. But I found this black tattoo like blob on his left ear. What is it?
I don’t really know if it’s an tattoo or a marker. But I did read online that vets like to put a M to say to people that his male.. is that what it is?? But why doesn’t it look like an M then.
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