I've had plenty of people reviewing my works and... I read somewhere that reviews aren't for the authors to read, it's for the readers or new readers.
Well, what I wish to say is that I think it's absolutely fine for me if you review people's work.
One thing I've noticed quite a lot is that people rarely review the ART, but more how they choose to tell the story and how.
For youtube I'd say: Ask first, just to be considerate. Some people won't mind it, but new artists (as well as oldies) might not want to be found on a site as Youtube (which... kinda is famous for people being mean towards artists in general).
As for blogs (which are lesser found on the internet) I'd say go for it! Don't bother asking, as long as you don't earn money on the content.
Show small snippets from the comic if you'd like, just to showcase what you think is the best appeal from the comic.
If you want to review, and wish to make the artist/author read it, be sure to give constructive criticism (what didn't work and how it could be changed for the better).
Sure, some people might not at all like what you're writing, but... It's just different opinions, and we all have those ;D
In this case I'd say that a review is a promotion whether or not it has a good or bad review.
Either could make the reader feel like: Oh man, this sounds really awesome/terrible, I must find out if this is true and... The author gets a new reader.
(I've been like that with movies, you know... The Room? Birdemic? Those movies are extremely terrible and so good at the same time but they were promoted as really bad budget movies and they ended up being really popular for a time!)