I have a purple bell shaped perennial growing in my garden, and I have forgotten the name of it.
It is not a campanula. It grows like a weed and I have seen it growing between blacktop and a building. It grows on a stalk.

Looks like Adenophora stricta to me. I call them Lady Bells and pull a lot of them out to keep the amounts down.

And we don't have any pictures of that in our database. Hint, hint!

How do you know it's not a Campanula, and specifically, Campanula rapunculoides?

There is a way to tell Campanula from Adenophora, which involves an easy dissection of the flower base.

It could be. You can let her know how to do that.

For comparison: Creeping Bellflower (Campanula rapunculoides)

If it is actually an Adenophora (yet to be confirmed), compare also to these entries:
Ladybells (Adenophora liliifolia)
Ladybells (Adenophora potaninii)

I've looked to try to find a key for Adenophora with no success. Edit: Oops, no, I have finally found one - http://www.efloras.org/florata...

So, what I'm trying to say is that the photo in question - a fuzzy one which shows only flowers - probably doesn't show enough distinguishing characteristics for any sort of a conclusive ID.