Salmos 105:28

Darkness. — The enumeration of the plagues omits the fifth and sixth, and begins with the ninth, and appends a clause which, from the first, has troubled translators. Of whom is it said, “They rebelled not against his words”? Of the Egyptians it is not true; and to refer the words to Moses and Aaron, in contrast with their resistance to the Divine command at Massah and Meribah, is feeble. The LXX. and the Syriac solved the difficulty by rejecting the negative. (Comp. the Prayer Book Version.)

The simplest explanation is to take the verb as imperfect subjunctive: “He sent darkness, and made it dark, that they might not rebel against his word.”
But this fails to supply a reason for the position in the list of the ninth plague, and the suggested emendation of Mr. Burgess is so satisfactory in this respect, that it almost by itself carries conviction with it. By a very slight change, he obtains: “He sent darkness, and darkened them, that they might not discern his tokens;” taking deber in the same sense that it bears in Salmos 105:27.

Thus the plague of darkness is, by a slight device of the poet, made to symbolise the moral blindness displayed by the Egyptians throughout.