how to set up a blank board?

You can do that on the analysis board on the website.
If you are using a laptop or computer, click on the mini chessboard icon on the top left side of your chat box, then click on set up position. From there, you can set your preferred positions. You can make moves from the starting position or start from a certain position. You can set castling to be legal, black to move etc. You can also input sidelines to a line if you wish. The following example, for example, shows the sidelines in grey in brackets.
Hi @eric002,
Could you describe in a little more detail where to find this, please? I'm not sure what you mean by 'chat box', but I can't seem to find what you're describing?
Many thanks
Mark

https://www.chess.com/analysis
Click on the above link and check the 'self-analysis' box, that should fulfill your wishes

Yeah, just click https://www.chess.com/analysis and click "set up" then click the trash can symbol at the right to clear the board, or just enter in moves from the starting position. If you're using a mobile app this may not work, if you're on the iOS app feel free to ask me.

Yeah, just click https://www.chess.com/analysis and click "set up" then click the trash can symbol at the right to clear the board, or just enter in moves from the starting position. If you're using a mobile app this may not work, if you're on the iOS app feel free to ask me.
Many thanks all.
What I'm after is just a plain virtual board, and actually it's specifically the analysis scores/lines that I don't want! The context is that whilst playing games I'd like to play out possible variations 'on the board' but deliberately without seeing (via the evaluation bar etc.) which are weaker/stronger - so that I'm not cheating, whilst I continue developing the skill of doing all this in my head.
In fact I think I've just realised what Eric originally mentioned (literally staring me right in the face I typed my original query!) via the chess board icon alongside the smileys/italic/bold etc. chat text features.
I think that will serve me.
If there's a better way to do this with a bigger board I'd be grateful, but at least this'll get me by.
Thanks

Many thanks all.
What I'm after is just a plain virtual board, and actually it's specifically the analysis scores/lines that I don't want! The context is that whilst playing games I'd like to play out possible variations 'on the board' but deliberately without seeing (via the evaluation bar etc.) which are weaker/stronger - so that I'm not cheating, whilst I continue developing the skill of doing all this in my head.
In fact I think I've just realised what Eric originally mentioned (literally staring me right in the face I typed my original query!) via the chess board icon alongside the smileys/italic/bold etc. chat text features.
I think that will serve me.
If there's a better way to do this with a bigger board I'd be grateful, but at least this'll get me by.
Thanks
On the apps, there is now a 'Pass and Play' option. It provides a live board for play in real life.
Thanks @notmtwain
SOLUTION: I've just discovered the actual "Analysis" button in the main gameplay screen itself (alongside the First / Previous / Next buttons) does exactly what I want - I figured there must be some place to do this
NOTE: That is playing via the webpage, I haven't looked on the iOS/Android apps yet.