I reasoned out this way of looking at it, please comment if you think it hits the spot or if it's way off in some way.
If you separate things into "Source of Damage" (henceforth SoD) and "Damage Type" (DT), you could in this case argue that when an effect causes enough damage to generate blasts, the damage from those blasts have a new SoD but they inherit the same DT.
an example: Rasputina casts December's Curse on a target. When it does damage, SoD is a ranged strike spell, triggering Bulletproof, and DT is magical so also triggering Magical resistance. If the damage was high enough, it results in Blast Markers coming into effect. When they do damage, the damage's SoD is Blast, so triggers Evasive (but not Bulletproof), but it inherits the DT of December's Curse being a spell so is still magical and triggers Magical Resistance.