The Hebrew does not have a separate word (thick or dark) acting as an adjective modifying "cloud". Instead there is a single word, עֲרָפֶל (araphel) which occurs just 15 times in the OT: Ex 20:21, 4;11, 5:22, 2 Sam 22;10, 1 Kings 8:12, 2 Chron 6:1, Job 22:13, 38:9, Ps 18:9, 97:2, Isa 60:2, Jer 13:16, Eze 34:12, Joel 2:2, Zeph 1:15.
The BDB listed meaning is:
עֲרָפֶל noun masculine cloud, heavy cloud (perhaps from above √ + ל
afformative; Late Hebrew = Biblical Hebrew; Syriac ; ᵑ7 עֲרָפֵילָא;
Mandean ארפילא NöM 126, 128; on vocaliz. see BaNB 160 and against him
SchwZAW x (1890), 178 who proposes עַרְמֶּל); — ׳ע in which God dwells
Exodus 20:18 (Gi; van d. H. Exodus 20:21), 1 Kings 8:12 2 Chronicles
6:1; + עָנָן Deuteronomy 4:11 (also חשֶׁח), Deuteronomy 5:19; Psalm
97:2; Job 22:13 see בַּעַד; תַּחַת רַגְלָיו ׳ע 2 Samuel 22:10 = Psalm
18:10; as swaddling-bands of sea Job 38:9; figurative of misery Isaiah
60:2 ("" חשֶׁח); of ׳יs judgment Jeremiah 13:16 ("" צַלְמָוֶת), as a
past day ׳יוֺם עָנָן וַע Ezekiel 34:12 = (of future day of ׳י)
Zephaniah 1:15 ("" יוֺם חשֶׁח וַאֲפֵלָה) = Joel 2:2 ("" id.).
Thus, the meaning is clear - it is a thick, heavy cloud that either obscures light, or, creates gloom, eg, Deut 4:11, 5:22, etc, depending on the context.
In 1 Kings 8:12, this makes the meaning easy to determine. The thick heavy cloud obscures the visage of the the LORD but does not necessarily imply darkness. On the contrary, we find in 1 John 1:5 -
And this is the message we have heard from Him and announce to you:
God is light, and in Him there is no darkness at all.
Thus, I understand the thick cloud ensures that God remains unseen (John 1:18), but not in darkness.
