For blastocysts, more clinics use the same grading system, which is usually a number followed by two letters. The number refers to the stage of development, not really the quality. If your blasts were 3AA and 4AA, then that is great. If they were 3CC and 4CC, then that is not as good. They would rather transfer a 3AA than a 5CC. This page explains the grading system well: IVF Blastocyst Pictures & Blastocyst Stage Embryo Grading Photos
An embryo that survives to a 5-day transfer has a better chance than a 3-day transfer, so you can feel pretty good about your chances. Though the numbers on this page are just for that fertility clinic, they give you an idea of what your chances are: 5 Day Blastocyst Transfer Success Rates Are Higher Than Day 3 Transfer The live birth rate is around 70% for under 35 and around 50% for 35-40, which as you can see is around double the odds for a 3-day transfer.