'First Blossom Open Date' is a simple user-defined biofix input that activates model predictions. As long as you have open bloom on any variety, the bloom date need not be set too early. If you have multiple varieties, start with the earliest cultivar's bloom date. Then, reset the date to align with the latest cultivar blooms, to ensure the model predictions do not end too early.
Note. Varieties that are not flowering need not be sprayed for blossom blight.