What would white blisters on tounge and sore throat mean i already went to local clinc and tested for strep and it came out no?

Virus or other: The doctor who actually examined you would be the best one to pose this question to. It could be a virus or something else. Have your parent call the office and request further clarification. Some things just recover on their own while others require further testing.

Infection: There are viral and bacterial infections other than strep that can cause this. Probably would be best to see ear/nose/throat specialist.

Hard to say: Need to have it checked out again. Could be a yeast infection?

Blisters: There are a number of things that could be responsible for blisters on the tongue, best way to attack this is to get a CBC and full history taken to see if anything tips off why it is there.