Blindsight and truesight

[Doomwhispo]

Can either see trough walls? Say behind full.cover?

Could truesight see te original form of a creature that has been true polymorphed years ago and is now stuck in a new form?

Is there anything you can do against truesight? To like fool it or is it by definition foolproof?

[Saggo]

1) No, though DM might decide otherwise in special circumstances.

2) Yes.

3) You can't fool truesight, if they can see you. They can't see through heavily obscured if it's not darkness-based, such as with Fog Cloud.

[Gignere]

You can cast non-detection to defeat true sight gained from the true seeing spell. Normal smoke, fog from spells or natural will block true sight but not blindsight.