Pines are one of the most difficult of all trees to bonsai because of the need to reduce candle growth in a uniform way. I have no experience with Pinus nigra the Austrian black pine but my tree encyclopedia informs me that they are the most durable of the European pines. I would think that they must be pretty forgiving in bonsai culture. That being said what do you do? In Japanese black pines P.thunbergi, this is the season to thin candles and pluck old needles. You should pluck off all of last years needles and remove most of this years as well leaving three to five bundles on each twig. Leave fewer needle bundles at the top of the tree and more as you get lower in the tree. This will help balance the strong top growth that pines usually have. Thin out the candles that formed last summer down to two candles on each twig. The candles that are left should make a horizontal V so that the new growth is horizontal rather than vertical. This thinning of needles will allow light to get into the interior of the tree and encourage back budding. The candle thinning will also reduce the apical dominance from the top of the tree and encourage back budding.

Thanks for your help.
I just saw an article that confirms what you have advised.
Treat the tree like a Japanese black pine.
I hope it works.
Paul