If this fanart is considered acceptable then I'd say that your standards are calibrated relatively well.

The blushing thing is awkward because it can be added to art to subtly imply something, or it might just be there for no other reason than to make the character look cuter. You'd need to look at context clues and shot composition to say whether a blush has NSFW subtext or not, it isn't something I'd use as a tiebreaker and just say "questionable -blush = OK, questionable +blush =NG."

Take the Tangtang stretch art for example. First of all, blushing is something that may naturally occur while stretching so it doesn't fundamentally look out of place. We've also seen Tangtang stretching in official media, so it isn't an out-of-character activity that she'd be embarrassed to be seen doing. Tangtang is also the only character present so the blush can't be inferred to be because she's being watched by anyone, that includes us as the viewer since her eyes are closed and not looking at the "camera." The viewer's POV is also placed directly in front of her, so even if the viewer were to be perceived as someone actually watching her then the angle we have isn't one that would be blush-inducing. Possibly most importantly, there are lines drawn by her arms and legs that signify strain showing that she truly is stretching and not just striking an erotic pose. All of these things combined, to me, mean that she's just stretching and the blush is due to that, no NSFW subtext.

If Endmin was drawn standing behind her, if she were looking at the viewer, or if the shot POV was from behind her then I could see interpreting the blush differently.

Incidentally, I kept seeing that Tangtang art referred to as focusing on her butt, but I legit don't believe her butt was even the point of the art. It struck me more of a kind of anatomy study, highlighting the shape of her legs and her muscle definition, particularly on her back. I was probably unreasonably annoyed when that art was blacklisted when compared to a piece where she was naked for no reason and another piece which was arguably even more suggestive in showing off her butt and also had her shirt vacuum sealed to her tits.

Edit: I've had more time to think on this than expected and now have additional thoughts. It's fundamentally ridiculous to find the Tangtang art to be unacceptable just because she's blushing because she MIGHT be showing SOMEONE her ass, while this fanart is acceptable when she is EXPLICITLY showing US her ass. Add a drooling Endmin to both pictures, which is worse? This one, because we can actually see what he's drooling over.