Yup, agree with Erik. Monoblocks have completely separate power supplies, whereas most stereo amplifiers used a single shared power supply. However, some stereo amps are described as "dual-mono", which are actually two amplifier channels with two completely separate power supplies and transformers (just mounted in the same case). Some stereo amps have a pseudo dual-mono design where you still have a shared transformer, but the left/right power supplies use different secondary windings from that transformer.
Also, monoblocks are usually, but not always, a fully balanced/differential design (i.e. push/pull). Some stereo amplifiers are balanced/differential, but 90% of the stuff out there is still single-ended.