Lovely blue - but not familar, I'm afraid. I hope someone else recognises it!

That's a Phacelia campanularia, but it's not a perennial. It does, however, reseed easily in fast-draining soil.

Isn't that used as green manure in some areas?

Spot on Tugbrethil, thanks a lot for that. I assumed perennial because I had 'Mertensia' in my head. Don't know why, they are not even that alike.
I knew I could rely on GOY.

You're welcome, Vincent. Spritz, I think that the one used for green manure is Phacelia tanacetifolia--a pretty plant in its own right, but quite different in appearance.

Thanks, Tug. I thought it was 'Phacelia'. :-)

Often sold just by the genus name, which gets confusing, fast!