Short answer, as I understand it-
Valvetronic is a throttless intake system, so no vacuum exists in the N55 intake tract. You need a throttle plate to make vaccum. Blowoff valves need a vacuum signal to operate. That's why on the 135i it is an electronic component, built into the turbocharger.
That said, "blowoff valve" isn't really what they are. The actual name is "compressor bypass valve," but no one will know what you're talking about if you call it that.