Time ago I've discovered a simple command line to turn the terminal window in a blackboard for just to take temporary notes. Talking about blackboard I doesn't mean nothing graphic, just alpha-numeric input. I don't remember that command anyway and hard googling didn't help me. Any help?
nano? That's a text editor, so you can read and write text files with it, but it can also be used for what you describe.
There is also the option of clearing the terminal and keeping it blank, making it totally empty and letting you write whatever. Something like
reset; cat > /dev/null would work. Note that you can not edit your "blackboard" this way, while with
nano you can.
If you do
stty cbreak -echo; cat, you can move around with arrows and clear the screen by typing
ESC [ 2 J, set colors by typing other ANSI escape sequences, etc. Exit with ctrl-C. You may need to run
stty sane afterwards, it might be worthwhile to set up a script to do this if you need it often.