Shadow Pokémon are a thing now. If you see a black mist over the top of a stop or it completely black; it means that team rocket has taken it over The stop will be completely black when you get in spinning distance. Spin and team rocket show up. If you battle them, you get to catch a shadow Pokémon. The battle is pretty easy but you will need to heal your Pokémon after. If you win, you get to catch the Pokémon. Afterwards you can purify it if you wish This uses stardust but it makes the Pokémon become level 25 and improves each of its iv stats by two. If it will show up right, here’s a pic of shadow Pokémon. If not, I’ll put in the comments.

ETA: made two comments with more info