Do you mean the Blackstar "amp" or "preamp"? Are you saying you have two preamps, or just the Blackstar amp and the Boss preamp?

If you're using a preamp and the Blackstar amp, you should be able to go guitar -> preamp -> Blackstar amp then simply disengage or bypass the preamp when you don't want to use it.

Does that help? Perhaps I'm not understanding you're situation.

Let me know.