I've seen on the Internet both that Bloomsweet is a cross between the pummelo and sour orange, and also between pummelo and sweet orange.
Millet, there was a DNA marker analysis study in Japan that showed kinkoji probably resulted from a cross between buntan (pomelo) and kunenbo. Bloomsweet is either believed to be the same variety as, or possibly a seedling of, the Japanese variety kinkoji. Another DNA marker study indicated kunenbo to be one of the parents of Satsuma mandarin (along with kishu as the other parent).
There were several Japanese citrus farmers who immigrated to Texas, so that is how this variety probably arrived in the U.S.
I'm not sure that Bloomsweet qualifies as a "real" grapefruit however. The blossoms don't smell anything like the blossoms of grapefruit or pomelo. It smells almost like neroli, or an intense bitter orange fragrance. (I don't mean the fragrance is bitter, I'm referring to the variety of orange)
Kunenbo is a large size mandarin, supposedly with a distinctive noble scent and very flavorful. Although it is a mandarin it probably has some small amount of pomelo going back in its ancestry.
Apparently it was widely known in the Edo period, but later fell out of popularity because it does have seeds.