Connecting an Interaction block to a Source block allows you to play that Source block, typically controlling pitch (frequency in Tone blocks, and playback rate in sample-based blocks). The only Source block which cannot directly be controlled in this way is the Microphone block.
The Computer Keyboard block lets you use your computer’s QWERTY keyboard as a music keyboard.
BlokDust supports MIDI with the MIDI Controller block, allowing you to use an external keyboard or other style controllers which are attached via USB. This uses the new Web MIDI API.