Wiring for ecobee3 lite
You have a bigger problem than chasing down a blue wire here

Here's the rub -- the Honeywell L8148A doesn't even provide a C terminal for the blue wire, PEK, or whatever to connect to, as it was designed for dumb mechanical 'stats.  Considering it not only contains a transformer and relay, but a high limit function as well, this leaves you with limited options -- simply replacing it with a separate transformer and relay won't do.  One could add another transformer and relay as an isolation relay, but that's probably more complexity than is called for here, considering that Honeywell makes a more versatile version of what you're looking at, called an L8148J.

Replacing your L8148A with an L8148J configured for a 24VAC gas valve (valve from B1 to B3) lets you connect the C wire to the TV terminal, jumper T to TV, and then pull the jumper from Z to W, connecting R to Z and W to W.  Of course, this all presumes that the cable doesn't transition from 3-wire to 2-wire somewhere in the middle; if it does, then you need to use a PEK or add-a-wire device to handle the situation atop the aforementioned aquastat changeout.