I've come up with an unorthodox mod that makes the blend useable in series mode, as well, if you're interested. It works in theory, as far as I can see, but I haven't tried it out, so I can't vouch for its overall practicality.
It does demand that the blend pot is not of the "both full at centre position", though, meaning you can't get "full" volume out of it.
On second thought, that might be solved with a few extra poles on the series/parallel switch...
*goes off to think*
Nope, that addition won't work.