I assume you mean Wittia amizonica, a lavendar-flowering Rhipsalis? Not blue, but lavendar. Not heard of any blue-flowering cacti.

I know this doesn't quite count, but after flowering, Cipocereus bradei has striking blue-black fruits.

Thanks anyway, both of you.

PS: Is Cipocereus bradei easy to grow?

Wittia amAzonica was described in 1903. There's no Wittia amIzonica.

-R
PS "lavender," not "lavendar."

I don't grow it yet, but I'm thinking of starting. It's one of Huntington Botanical Garden's International Succulent Introductions for 2011 for $6. They write: Cipocereus is a genus of just a handful of species of columnar Brazilian cacti. Its distinctness as a monophyletic group is supported by DNA evidence as well as seed and stem morphology. In C. bradei the spines are lacking or few and when present are short and black or dark brown. Therefore, it is the inflated-looking, glaucous-blue, clavate stems that are most characteristic. The nocturnal flowers have pale petals which surmount fleshy tubes and develop into dark, globose fruits that appear blue by virtue of the thick cuticular wax covering the black flesh. This species was originally described as Pilocereus bradei Backeb. & Voll and for a time was called Pilosocereus bradei (Backeb. & Voll.) Byles & Rowley. We offer HBG 104728, seedlings from plants in cultivation; however, the species is known only from the area around Diamantina, Minas Gerais, Brazil.

Here is a link that might be useful: ISI 2011-1. Cipocereus bradei (Backeb. & Voll) Zappi & N. P. Taylor

PS Gosh... Going from your username Ariocarpus78... If you already grow Ariocarpus successfully, I'm guessing from the lack of culture cautions for Cipocereus bradei (in a cursory web search) that you will not have too much trouble.