Certain STDs and STIs can be transmitted through the thin lining in the mouth, this is more likely if the person who gave you a blow job had a cut or sore in their mouth. Most commonly these are Chlamydia and Gonorrhoea, but can also include HIV although that is rare.
Better2Know would suggest that you have Chlamydia, Gonorrhoea and an HIV test. If you think the person who gave you a blow job had a cold sore then we would also suggest a herpes test.
Everyone’s circumstances are different, and it is your responsibility to choose the tests that are right for you depending on your sexual activity and what you know about your partner. If you are unsure then Better2Know recommends a Full STD screen. Once is enough to catch an STD, STI or HIV.