List of free blackboard stuff for developerππ½
If you're looking for all programming tools around blackboard (not just free). Check this page List of blackboard Programming Tools

Onlineboard
Browser based online whiteboard app. Allows you to create a whiteboard where others see everything in real-time as you draw it. Dark mode, no login required, multiple colors, export to PNG.
Tag: remote, productivity, whiteboard, blackboard, multiplayer, collaboration
Dark mode Zoomable board Up to 32 users No signup required Export to PNG Follow your guests on the whiteboard