Assalamu alaikum, Somebody told me that jinns take the form of black dogs and that there is a certain verse of surah Al Baqarah that one should recite then spit on the dog, then it will never bark at you or something like that. Aren't they just dogs, or are they jinn? Jazak Allah

I dont know any authentic proof for what you have mentioned. Not every black dog or animal is a Jinn, but sometimes the Jinn can take the shape of black dogs or cats. The default ruling is that the black dogs and cats are just dogs and cats.