psalms 105:28 He sent darkness, and made it dark; and they rebelled not against his word. - Free Bible Online

      28-36. The ninth plague is made prominent as peculiarly wonderful.
      they rebelled not--Moses and Aaron promptly obeyed God (Heb 11:27); (compare Ex 7:1-11:10 and Ps 78:44-51, with which this summary substantially agrees). Or, rather, the "darkness" here is figurative (Jer 13:16), the literal plague of darkness (Ex 10:22, 23) being only alluded to as the symbol of God's wrath which overhung Egypt as a dark cloud during all the plagues. Hence, it is placed first, out of the historical order. Thus, "They rebelled not (that is, no longer) against His word," refers to the Egyptians. Whenever God sent a plague on them, they were ready to let Israel go, though refusing when the plague ceased.
      his word--His command to let Israel go [HENGSTENBERG]. Of the ten plagues, only eight are mentioned, the fifth, the murrain of beasts, and the sixth, the boils, being omitted.

JFB.