Is it my responsibility to point out that a paper has been plagiarized from another researcher's blog? While reading a series of blog posts on a sub-field in TCS (theoretical CS), I came across a paper recently published by a group of students/faculty that has literally copy-pasted large sections of a blog post, and made it into a journal paper thats available online. My question(s) are about what I should do about it:


Do I point it out to the author of the blog, and leave it to him
about how to handle the issue?
Contact the journal in question - but since its not my work, I'm hesitant that whether I have a case 
Leave it alone, finish reading the blog posts, and add the journal to my private list of journals to completely ignore?


The third option is most hassle-free for me, but I wondered if it'd be ethical to know about plagiarism and not point it out!
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.)