Campanula lactiflora flowers for a good part of the summer/ Mine is just coming to an end now, but if where you live would allow a 'Chelsea chop' you might be able to delay flowering, though I've not tried this myself. My lavender blue one was really dark this year with the weather we've had.
Geranium 'Rozanne' is a lovely blue and flowers all summer. Though it is not itself a tall plant it is very happy to scramble up in its neighbours and show its face higher and wider than you might expect
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