Asters usually last a couple years in my garden if we do not have a winter like we had last year. The way I find the blue ones is checking all the nurseries this time of year. They usually have blue. I found that if you find a really deep purple one and plant it with the purplish ones it makes the blue one really stand out as blue.
Do a eye trick.
I picked this one up at the grocery store last year and it was close to blue.
I planted this one next to it and it made the almost blue one look bluer.
