Blue Hubbard is a Cucurbita maxima. I thought Butternut and Tahitian were both C. moschata, but some pages say Butternut is actually C. mixta. Maybe the species have been redefined since I was in school. Anyway, as far as I know, the different species can't cross. I don't know what the barrier is.

Butternut is a moschata and is crossable (not so easy but possible) with C.maxima vars if used as pollen parent. Tetsukabuto is from such a cross.