Nope. Well, technically there's the tiniest of possibilities that you could have cross-species or cross-genera pollination but in reality this just doesn't happen. The idea of species involves interbreeding. And thinking about this a bit more, there's nothing particularly special about Bocking 14 other than the fact that it is sterile, right? There are lots and lots of plants out there that utilize pollen to reproduce and pollinators like my beloved honey bees fly about from clover to flowers to veggies and most definitely transfer some amount of pollen from one type of plant to other plants without any cross-pollination taking place. Plus, wind pollinated plants (like corn and many trees and grasses) spread pollen all over the place without cross-pollinating the tomatoes or cukes or beans.
I suspect that pollen from Bocking 14 might cross with some other non-sterile comfrey and the resulting seeds could be sterile or not and might have some mishmash of characteristics of traits from those varieties. But folks probably don't plant multiple varieties of comfrey if they are planting Bocking 14.
Interesting idea of developing a sterile comfrey to control its growth. I've never grown comfrey but I imagine it could get out of hand if left to its own devices. Mint can get out of control too.