Torches don't block spawns. The reason you don't see them being used is two-fold:
1)though the difference is only 1 block, torches produce less light than block lights
2)torches have to be placed "in the way", which means active-flushing mechanics will generally knock them off (torches get washed away in water, get knocked off by slimeblock sweepers, and shaken off by shifting floors [a mechanic that got patched away fully in 1.14 when even soul sand stopped working]).
To answer your title question: in 1.14 all mobs spawn on transparent blocks, but I'm not sure if this is intended or just a bug waiting to be fixed (or something that was only recently fixed). In 1.13 and lower, no mobs will spawn on transparent blocks.