30 feet is possible, but you’d be very very old by then, as almost sure they grow just as slowly(or just a little faster since you have a longer growing season and get more rain) there as they grow here in las vegas.
i have two noirs, acquired them ~3 years ago at less than a foot tall, and they have yet to reach 2 feet tall.
and speaking of your intent to have them at 8 feet tall, i bit the bullet and bought two 7 to 8 footers(persian blacks, not noirs, but taste and lack of vigor are the same) last december. Very productive this spring, and though summer is almost over, none of the branches have grown longer than 6 inches…
am a huge fan of black mulberries too. Among fruits from deciduous trees which may be grown in continental usa, totally agree with some publications and posts at fruit forums that blac mulbs are the best-tasting.
ms whitman of whitmanfarms.com(major supplier of noirs) sent me this winter pic of the ‘mother of all noirs’, which was >10 years but look really runty. It is the dark and gnarly tree in the foreground
here is a thread with some pictures of my noirs