I've sometimes heard people say, 'confound it'. So, I'm wondering whether this is considered a euphemism.
To some extent this is a matter of personal opinion. I think "confound it" is just another mild interjection to express frustration, like "darn!" or "drat!" or "rats!" Meanwhile, a euphemism is a "nice" word that is used in place of a "not nice" word, such as saying "darn it!" instead of "damn it!" (which in some social situations would be considered an unspeakable expletive).
It's possible that some people say "confound it" as a mild substitute for a stronger expletive such as "fuck it" ... but no one I know does this.