Well, The Font of All Knowledge (a.k.a. Wikipedia) tells us several things:
- There is not much similarity as far as phonology, as Hurrian seems to lack consonant voicing distinction (except in certain circumstances). BS clearly has this distinction.
- There's not enough BS to determine much by way of grammar, but Hurrian seems to be extremely agglutinative; BS doesn't appear to be at all.
- Hurrian seems to have more vowels.
- BS has more consonant clusters.
Upon reading this analysis of Black Speech, I think it is clear that Professor Tolkien must have taken some words & grammatical forms into BS from Hurrian. For example:
- -at < Hurrian -ed- formant of jussive/intended future in verbal form formant of future in verbs
- -ûk < “All”, “completeness”; Hurrian -ok, formant with a meaning “fully, truthfully, really” in a verbal form.
Hurrian texts were known since discoveries in the 1910s & 1930s. A grammar wasn't published until 1941. Based on the Analysis, I think it very likely that Prof. Tolkien gained inspiration from the Hurrian direction. I'm not sure I'd agree with "strong similarity" based on the extreme paucity of BS evidence. More evidence, a lost grammar, those would be more convincing one way or the other.