No, they are NOT a true blue. That color doesn't really exist in Hibiscus. They are a lavender/lilac as you say. The marketing people just take liberties in this area.

"Blue" impatiens are also the same lavender/lilac shade, as are "blue" roses (except, of course, for the white ones that are dipped in blue dye for the floral trade!).

This is a Rose of Sharon, Bluebird variety. It isn't exactly blue either.
Rose-of-Sharon, Bluebird
{{gwi:892433}}

Thanks for the post as they are beautiful.