Vbenny, lots of products say that they are "for flower/fruit", or for "rooting". It's just NPK after all. I compare this to the aspirin industry. They have ads for AM, PM, PMS, back, front, head and every other ailment imaginable, yet it all contains the same ingredient. It's just fucking aspirin!! (or ibuprophen)
I would think that any fert with a high P content would be good for rooting, AND flowering. These micro beasties are kinda the same. Late application certainly couldn't hurt, just that you'll only realize the full potential of benefits by incorporating them at the onset in your soil.