Electrical – Where is the C wire terminal on the boiler

According to the control wiring supplement for your boiler, there should be a C terminal on the transformer, adjacent to the R terminal. Look for the terminal on the transformer with the green wires going to it.

Look for the junction of the thin white wire and the two fat yellow wires -- that's where your C is

Since you have an air conditioner in this system, we can use the wiring going off to the condenser as a clue to find your C wire -- the condenser contactor/control must be connected between Y and C as the thermostat switches R to Y to call for cooling.

Following this logic, we start with the yellow wire in the cable from the thermostat (it's the cable that terminates at the top of the picture), which is connected to the red wire in the cable going to the condenser (the cable that terminates near the bottom of the picture). Since that wire is clearly Y, the other wire in the cable going to the condenser (the white wire, since it's a two-wire cable) must be your C wire, and you can connect the blue wire in the thermostat cable there (after turning the power to the furnace off and stripping the wire end back, of course).