So you think that wire is either constant 12v or ignition 12v then?

Is it safe just to run an extra wire to battery or ignition and see if that works? I see no way of finding out where that blue wire goes since its part of the harness and I'd have to tear the intake manifold apart to see where it went...

__________________
2016 Monsoon Gray Audi Allroad - 21k
2008 Black Mercedes E350 4Matic Sport - 131k
2014 Jeep Wranger Unlimited Sahara - 62k
2003 Gray Mercedes ML350 - 122k

That would be my supposition, yes. The blue wire is probably either +12 from the battery, or +12 from some switched source.

It is possible that it could be run from the compressor, by I'd speculate that whoever wired it was cautious about overloading circuits and would be less likely to tap off the compressor pull-in wire.

As far as safely tracing it back, I'd put a meter on it to figure out how it's powered (steady +12, switched + 12, compressor switched +12) and then check those places. I'd look at the battery terminals, and near the fuse box and see if there's a blue wire.