Feminine from an unused root meaning to cover; dusk -- dark, twilight.
Englishman's Concordance
1) thick darkness Part of Speech: noun feminine A Related Word by BDB/Strong’s Number: from an unused root meaning to cover Same Word by TWOT Number: 1626
Brown-Driver-Briggs
עֲלָטָהnoun femininethick darkness**< darkness, dusk, which suits both passages (Bevan, privately). (Late Hebrew עֲלָטָה (rare); according to BaES 5 transposes from Arabic [be cloudy]; darkness); — absolute ׳ע, attending sunset Genesis 15:17 (with verb masculine; AlbrZAW xv (1895), 325 KöSynt. § 350 e read הָֽיְתָה); ׳בַּעEzekiel 12:7 ("" בָּעֶרֶב), Ezekiel 12:6; Ezekiel 12:12.
עֵלִי, עֱלִי, עִלִּי, עֲלִיָּה, עֶלְיוֺן see עלה. p. 750f
עֲלֵי see עַל. עַלְיָה, עַלְיָן see עלוה, עלוז.p.752
Genesis 15:17 HEB:הַשֶּׁ֙מֶשׁ֙ בָּ֔אָה וַעֲלָטָ֖ה הָיָ֑ה וְהִנֵּ֨ה NAS: had set, that it was very dark, and behold, KJV: went down, and it was dark, behold a smoking INT: the sun had set very came and behold
Ezekiel 12:6 HEB:כָּתֵ֤ף תִּשָּׂא֙ בָּעֲלָטָ֣ה תוֹצִ֔יא פָּנֶ֣יךָ NAS: [and] carry [it] out in the dark. You shall cover KJV: [and] carry [it] forth in the twilight: thou shalt cover INT: on shoulder Load the dark carry your face
Ezekiel 12:7 HEB:בַקִּ֖יר בְּיָ֑ד בָּעֲלָטָ֥ה הוֹצֵ֛אתִי עַל־ NAS: I went out in the dark [and] carried KJV: I brought [it] forth in the twilight, [and] I bare INT: the wall my hands the dark went above
Ezekiel 12:12 HEB:כָּתֵ֤ף יִשָּׂא֙ בָּעֲלָטָ֣ה וְיֵצֵ֔א בַּקִּ֥יר NAS: [his baggage] on [his] shoulder in the dark and go KJV: upon [his] shoulder in the twilight, and shall go forth: INT: shoulder will load the dark and go the wall
[be cloudy]; 
darkness); — absolute ׳ע, attending sunset Genesis 15:17 (with verb masculine; AlbrZAW xv (1895), 325 KöSynt. § 350 e read הָֽיְתָה); ׳בַּע Ezekiel 12:7 ("" בָּעֶרֶב), Ezekiel 12:6; Ezekiel 12:12.