What is the correct wire size to run to a 12V 1.5 amp window blinds motor that is 47 feet from the DC transformer?
According to this site 18/2 SPT-2 lamp cord uses 18AWG wire, which according to this site has a resistance of 0.006385 &ohm; per foot. You are running 2 wires 47 feet so the loop length is 94 feet and the loop resistance is 94*0.006385 = 0.6&ohm;. 

Assuming the motor doesn't draw more than 1.5A the maximum voltage drop is 1.5A*0.6&ohm; = 0.9V and the voltage supplied to the motor at 1.5A is 12V-0.9V = 11.1V. It will probably work fine. The wire is rated for 10A so it should be fine too. 

The only thing that might be a problem is if the motor stalls, since there is an extra 0.6&ohm; in the circuit that could prevent a protective fuse from blowing. Hopefully the 'transformer' has over-current protection or is fused at a current not too far above 1.5A.