What does "ve'Yamesh Choshech" mean?
Rashi #2 citing Targum Onkelos, Seforno and Targum Yonasan: The darkness will descend after the darkness of night has been removed. 3
Rashi #3 (citing the Midrash) and Ramban (in Pasuk 23): The Pasuk means that the darkness was so think that it was tangible. 4
Inasmuch as it lasted for three days (Rashbam).
Like we find in Beshalach (13:22 [Rashi]). Only the 'Vav' in ve'Yamesh" is then superfluous (Rashi [See Targum Yonasan]).
Which explains why the Egyptians were unable to move, as the Pasuk relates (in Pasuk 23 [Refer to 10:22:1:1), and why Moshe had to raise his hand above the heaven to bring it down (Ramban), because the darkness of night, which is merely the lack of light, allows the light to dispel it; whereas the darkness of the plague was (a creation) so thick that light could not penetrate it. The Ramban, citing the Ibn Ezra, suggests that it was a thick fog (See Ramban, Pasuk 23). See Ki Savo (28:29 [Rashi]).