Wow - I thought I would try to look up a reference for you for when to prune your black pine, thinking I would find something that said prune in spring or prune in fall or whatever. Turns out it is way more complicated than that, with several different kinds of pruning that are done at different critical times.
"I discourage beginners from working with Black Pine, Pinus thunbergii, because it takes so long, and you need to know so much just to begin. I started with Black Pine, ignorant of this fact and massacred a lot of plants learning how, even with John Naka's book, Bonsai Techniques I. I bought about 100 of them in one gallon cans, old root bound plants that appeared to have potential. Some of them have turned out to be really nice trees, but ten years, later the best are still several years away from being finished bonsai. Not a subject for the impatient. So when beginners ask me what to do with this seedling pine they just bought, I just roll my eyes and try to talk them into a nice deciduous tree."
That's the introduction to that article, then it goes on to give a lot of info.
Bonsai4me has an article on pruning pine bonsai:
which also starts out with a discouragment:
"Pine pruning techniques in reality are very straightforward, however trying to learn them can be very confusing as there is so much contradictory advice offered in Bonsai publications and books. This confusion normally arises from the attempt of Bonsai publications to be too specific about the precise time of the year that certain techniques should be carried out. Unfortunately, different pine species require pruning at slightly different times of the year; different climates will also affect the advancement of Pine growth through the year and this also causes creates confusion when trying to follow advice that has been written for a different climate. "
But it looks like really good info.
I have never tried pine bonsai and haven't had real good luck with even growing full sized conifers either in containers or the ground, so all I can say is you should do a lot of reading and preferably find a bonsai club/ class near you before you start chopping in to your tree. Bonsai is an art for the patient. If you end up waiting until next year to do anything but learning how to take care of your tree, that's probably ok too....