Is this bait for getting justification for making sweeping statements about groups of people? Or am I just in a grumpy mood.

Depends on how wrong you're willing to be.

You can just say you have blue balls, nobody will want to look.

70% or so

With no additional context, if you said that "the balls in this glass are generally blue", I would interpret that meaning every ball falls within the range of hues that can still be called blue by most people but may be questioned by a few. So 100% of the balls have to be "I can see why someone would call that blue".

This is actually an interesting question that depends a lot on a human perspective and what you define as ‘Generally Blue’ and what’s else is in the container.
I personally would define generally blue as when you have two colours, say red and blue M&Ms, and you reach in the container to pull out 3 you have a 50/50 chance of pulling out three blues. This definition would require that you have 79.58% blue M&Ms in your container.
Honestly this sounds like a bit high to me but probably can be a real bitch, and the test sounds secure to me. Physiological I think 2/3 blue 1/3 red would have most people say it’s generally blue and the ratio would go down as you add other colours. I think 1/2 blue and 1/2 a mix of 4 or more colours people would still say it’s generally blue.

instructions unclear, painted my balls blue

Depends on how many balls fit in the glass, and whether you can see through the balls- if not, as long as all the balls against the glass surface and on top are blue, with a few more blue visible within the gaps should be enough. But what percentage that is could change according to ball size in relation to glass size.