Black Locust honey is also sold as Acacia honey here in the US & it is AWESOME. It's a very delicate & floral honey, I wouldn't waste it by mixing it with buckwheat honey. I've made mead from Acacia honey & while there is some loss of those delicate, floral notes during fermentation, there are enough retained to make for a fine mead.
If I were you, I'd make a medium show mead with that black locust/acacia honey. I don't think there's anything better you could do with that honey. As for the buckwheat, I think you'd do well to blend it with something more bland, like clover or alfalfa honey. Buckwheat honey has a very strong flavor & is often described as being "molasses-like" or as having a "barnyard-like" flavor. To my tastes, buckwheat honey tastes like a hay barn smells. Not bad, just a bit different IMHO. I think most people would be happier with a 50/50 blend, but your tastes may vary. Taste those honeys & think about how they'll taste post fermentation.
Out of curiosity & if you don't mind me asking, what did you pay per lb for the black locust honey?
Regards, GF.