I read somewhere that an 18/2 wire could be used to connect a transformer to my window blinds motor (12V 1.5 amps). However, it didn't say anything about the distance. Since I am running it inside the wall, I was looking at getting "UL and cUL Listed SPT-2 Wire" 18/2 electrical wire. Will the distance cause a problem?
According to this site 18/2 SPT-2 lamp cord uses 18AWG wire, which according to this site has a resistance of 0.006385 Ω per foot. You are running 2 wires 47 feet so the loop length is 94 feet and the loop resistance is 94*0.006385 = 0.6Ω.
Assuming the motor doesn't draw more than 1.5A the maximum voltage drop is 1.5A*0.6Ω = 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 0.6Ω 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.