Hi there.. In general i would suggest to use any bloom-nutrients or bloom-booster or similar once the stretching phase at the beginning of bloom has ended and the plant is putting its energy into creating buds. During the stretch it still needs nitrogen, which is usually limited in bloom-nutrients, so it's best at this point to still provide veg-nutrients (or a mix of both) until the stretch has stopped. Once the plant focuses on creating buds and the plant requires more P and K, which bloom nutrients are high in, or what's in bloom-boosters, then it's best to do the switch.
happy growing