I Kings 21:10 and 21:13 use a Hebrew reverse apophasis known as "sagi nahor" (שגי נהור), an Aramaic expression that translates literally as "great light" but refers to someone who is severely sight impaired or even blind.
Sagi nahor uses opposites or "clean language" where explicit language would either be considered uncouth or would violate a taboo. This manner of speech is common in both Hebrew and Arabic. It is almost always lost in translation because the linguistic norms of the writer are not explicit in the text and are not necessarily known to the reader. That is why the translations use "curse" or "blaspheme" even though the literal meaning of
the word is "bless". A native speaker doesn't even blink at this.
In the case of I Kings 21:10 and 21:13, as in Job 1:5, 1:11, 2:9, the taboo is making explicit mention of cursing God, because the explicit mention, especially in writing, is by itself a kind of curse, an apophasis. In particular the sensitivity is in using the word "curse" (קלל) juxtaposed to a name of God, particularly in writing. This is unthinkable in a text whose purpose is to praise God and show His mercy and justice.
In Leviticus 24:11, the story of the cursing by the son of Shlomith bat Dibri from the tribe of Dan, the MT is careful not to use any name of God, or even the word "curse", using instead, the word "mention" (ויקוב) and "the name", followed by a connective "vav" ("and") and then the word "he cursed", as if to say "He mentioned the name. And he cursed". The Cambridge NEB translates "the Holy Name" where the MT uses only "the name", and translates the whole verse "He uttered the Holy Name in blasphemy, so they brought him to Moses". This is a clear translation for the English reader but obliterates the circumlocution, the sagi nahor, of the MT.
In Leviticus 24:15 the word "curse" is juxtaposed to "his God" or "his deity" (אלהיו) rather than to a name of God. The following verse, 16, specifies the case of someone who blasphemes using the Holy Name. In this verse there is no mention of cursing. The NEB gives "Whoever utters the name of the Lord shall be put to death...", though it is clear that the meaning is "Whoever curses God using his Holy Name shall be put to death".
Many other OT verses use this type of composition. Here are three examples using circumlocution of the word for "unclean" (טמא, "tameh"):
Deuteronomy 23:10 (MT 23:11) (NIV):
If one of your men is unclean because of a nocturnal emission, he is to go outside the camp and stay there.
The MT does not say "unclean" (טמא), it says "he is not clean" (לא יהיה טהור). Here the circumlocution is completely lost in translation because the Hebrew words for "clean" and "unclean" are different words rather than a single word and it's negation as in English.
The same circumlocution of "unclean", with loss in translation, is in Genesis 7:8 and I Samuel 20:26.