שמות י,כא : וְיָמֵש חֹשֶךְ
The Sforno states that the natural darkness of night will be removed (dark only due to the lack of light). However, at this time that darkness will be replaced by a substance that can not receive light because of its great density (think of the dark hole).
The verb וימש is usually interpreted as darkness so intense and substantive that it could be felt. The Sfrono, however, interprets this verb from the hiphil of the root of מוש (departs or removes). Thus, the plain meaning of the verse is that the usual darkness of the night is removed and replaced.