I remember a comment here on Danbooru that was to the effect of "the ~ isn't used in English the way it is in Japanese, so translations should avoid using it", but I couldn't remember what the offered solution was. Anyone have an idea?

Not sure what solution you’re referring to, but as the tilde is used in Japanese to draw out the final vowel (“aoi no neeee”), the most sensible translation would be to draw out the end of the sentence in the translation too. In this case, you could write “bluuuuue”, but I think it’s more natural in English to draw out the preceding “really”. If you choose this option, make sure to draw out the vowel from the syllable that has emphasis on it. Drawing out the silent “e” of “blue” makes no sense and, for example, drawing out the “n” of “ocean” is just stupid because almost nobody would talk like that. I’m mentioning this because I’ve seen both of the latter variants in scanlation already. -_-,

Alternatively, you could go with the reasoning that drawn-out vowels, especially for the affirmative ね, is an idiosyncrasy of the Japanese language that has no equivalent in English, so it can’t be translated, but it can’t simply be left in either because nobody talks like that in English. Therefore, it should just be left out.

My solution is to decide between the two options on a case by case basis. If it makes sense to draw out a word in the English translation, like with the “really” here, I leave it in, otherwise, I just leave it out because I feel it adds nothing to the translation and just sounds stupid.