The bloom is meant for entire flower. The hpk is meant for late flower as a bloom finisher. Now they bothal are low as far as npk goes and im sure hpk woulf be fine..in fact i run a different line. Bio bizz and my bloom nitrients is 1-2-2. Its not enough phosphorus so i add a pk boost with another fertilizer. Id get both. That hpk will help alot.
I am using Roots Organics as well (only Grow and surge), the guy at my hydro store and like @Boognelson87 said the bloom is for all flower and the HPK is a finisher .
I got samples from Roots Organics of Both the Bloom,HPK and some microbes to test out. if you go to there site you can get 3 free samples just have to pay shipping which is $8 if you want to test before you buy .