Is there difference between having immunity to damage from sources of cold damage (given by Resistance power level 9) such as a blizzard and having protection from cold environments (given by Sustenance power level 3)?
I realize the sustenance only protects from the environmental damage but does the resistance only protect from damaging attacks? Does a naturally occurring blizzard count as an attack?
Yes, yes, maybe maybe.
Very situational, and dependent on the GM, the world/setting, and the campaign. A blizzard could count as an attack, it could just be environmental damage. The GM might require fortitude saves. Sustenance could counter it, as well as resistance.
Sustenance wouldn’t protect against an actual extraordinary attack though, but resistance could help while in a blizzard I’d say.
Whatever makes the most sense at the time and helps feed the narrative and fit what is happening.
Up to the GM, of course, but it seems fine to me to have a PL 9 boon copy some small part of the effect of a different boon. Per the rules as written, Resistance only protects against attacks, but if it makes narrative sense and the GM doesn’t see a problem then protection against other effects from that “element” are reasonable to allow.