"not delicious" does not capture the meaning of 맛없다. To me, "not delicious" though almost never used, would mean "OK" but not great. 맛없다 means it tastes bad or has no taste. I agree of course that this is a blasphemous statement, but on top of that, it is a statement almost nobody would use.
Yes, techinically "no taste" can be translated "not delicious", but part of the art of translation is taking the phrase from the base language (language being studied) and express it naturally in the target language while trying to capture the original intent of the speaker or author. When a Korean says "맛없다" the intent is almost always to convey that he/she doesn't like it, that it tastes bad, or is tasteless. You are not likely to hear an English speaker describe bad-tasting food as "not delicious."