Is it my responsibility to point out that a paper has been plagiarized from another researcher's blog?
Ignoring the journal in question is probably an overreaction.

However, the best route to take is the first one—the author of the copyrighted material is the one that has the most responsibility to assert her rights. Your duty is to notify the person whose work has been infringed upon that the violation has taken place. Once you've done that, you've achieved what's ethically required of you. You are not obligated to take the matter up directly with the journal. (However, you may do so if you so choose.)