Can I use this as a bloom booster?

I want to use it up and I’m wondering if it’s safe to add a little to my nutrient mix during flower.

It’s probably safe but counter-productive. during flowering you really want the plant to put its energy into flowering and not the roots.
Ages ago on a old site called overgrow someone explained it in more detail shame I can’t access that info no more as the site got busted in the late 90s and we all scarpered

Too high in P in relation to K. Something like 5-10-20 would be better. Or splurge and get yourself a small container of AN Big Bud powder. 0-15-35 with 10%S, 20 amino acids and ascorbic and citric acids.

I hate to flower without it. Comes in an organically derived version too.

I bought a couple of those. The basic one 12-6-6 and another I can't find right now for tomatoes then noticed they have zero trace elements which would be OK for a real soil grow maybe but gave them to the wife for the garden. Not the best for pots of promix.

Unless you know what the hormones are going to do *at that stage of growth* you should stay away from it. It's more likely to fuck something up.