After a few weeks of confirming its fondness of cat food, I trapped it in a cage with cat amenities, tried to pet it and got bitten.
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Scared and alert fluffy black cat on a perch in the upper corner of a big cage, at eye level with me and camera
The wound was shallow thanks to gloves and it is almost healed; I thoroughly disinfected it ASAP of course, with lots of scrubbing, water, alcohol and non-alcohol disinfectant.
No, I can't check for a microchip unless the reader works from a foot away (most are cca 2 inches), which would allow me to place it close to the food bowl. I could try to trap it again and check for sex etc., maybe even use a phone or generic RFID reader. It will indubitably get mad at me and I don't know if welding gloves will be enough to prevent more scratches.