to Vac_whisperer: Yes, it is a Tradition shake-out bag. I have had it for several years. I had been thinking about re-trimming one of my older Kirbys in dark blue(I have several 'bojack' trim kits in dark blue). That, and I just needed to put a bag on that machine for the short term. That 560 needs new bearings and a new brush roll at some point. If I put that much into it I will probably put a Heritage II mini-emtor on it with a new open-top bag(black) as I have done with my other Kirbys. I have a 500 series motor unit coming and it may wind-up being trimmed in blue and if so, it will get the blue bag and emtor.
As for being rare, that is my view as well. There are NEW blue shake-out bags, and I'd rather have one of those. They are very nice, and I have found a seller on eBay who has them for under $40. If I do retrim an old Kirby in dark blue I will, when I can afford it, put a NEW blue shake-out bag on it.
If you can't tell, blue is my favorite color.
Justin