Whenever I need some "custom" blend option, I just build a 2 channel mixer (
http://tagboardeffects.blogspot.hu/2013/01/2-channel-mixer.html), and use it's output.
If I need to have control over both signals, this will provide two pots for that. If I need just one pot to control the clean signal for example and save space on the enclosure, then I replace the other pot with a trimmer, and hide it inside, and that's it.
But that's just my way to do these sort of things :)