I'm with Anna, I think it's your call!
On the one hand, the amount of cartoony blood featured here wouldn't be considered NSFW in most other contexts. If that's a place you want to MAKE A STAND BECAUSE WE NEED TO NORMALISE IT then I think you could make that argument! -- you're just gonna make stuff a little tougher for your readers.
On the other hand, if I looked over the shoulder of my male co-worker and saw him reading a comic with a girl holding up her blood-stained panties, that'd be weird for me. I would probably not think "oh cool, he's got no hang-ups about this natural part of life!".... in fact I could easily see thinking "why is he reading a comic about girls' periods, is this some kind of....period fetish....." because I wouldn't have any context for what he was looking at. I might later realise that's a bad assumption to make, but not everyone would.
(Although if said male coworker sent the comic to me, that wouldn't be as weird; I would laugh at the comic and get a kick out of the fact that he's cool about this stuff. It's all about context.)
I think what you have is fine; the fact that it just says "contains blood" is enough to let me know that if I'm not weirded out by blood, that I can click this safely! And frankly, if you're concerned about sending the wrong message, you can always say in the description that "I know people have been dealing with periods since they were ten, but I can see it being awkward to read at work. xD"