What happens if i black list my iphone while someone is using it?

I lost my iphone a fw days ago they swapped the sim because its still on find my iphone but they didn't take off my icloud. I honestly for got my password, so what will happen if i just black list the phone through att will it like immediately stop working or will they still be able to use it cause thy swapped sims?

Your phone will have more bugs than Lindsey Lohan's tewat.