If blood work and ultra scan came out ok, can I be sure I don't have testicular cancer?

Yes. If a mass or lump was felt by a urologist then ultrasound and blood work are done. The mass can then be seen to be a cyst or not solid mass. If the ultrasound was not okay, the only way to know that its not cancer is to remove the testicle. Check with the urologist to see what was exactly seen on the ultrasound. It may need close observation and repeated ultrasound down the road to make sure.
Yes. Scrotal ultrasound is a very sensitive test for testicular cancer. If no suspicious tumor is seen, you can breathe easy. Tumor markers in the blood such as beta-hcg, afp, and ldh can help diagnose testicular tumors, but it is not as good a screening test as the ultrasound. Continue with testicular self exams, particularly if you have risk factors such as a family history or prior history of tumor.