They are totally different things, but can be used together with some intermediate software.
MQTT is a messaging service which allows easy communication between devices. This might mean a device sending temperature readings to the MQTT broker to be picked-up by another device (maybe attached to a relay to turn a heater on and off).
Blynk’s main feature is the fully customisable app, which can be used to control devices directly, or (as I choose to do) as a front-end to an MQTT based system. To make that work effectively, I use Node-Red as a sort of ‘middleware’ system.