The Yerushalmi, in Pesachim 1:1, says that if one is in a dark cave and cannot discern when it is night, he should ignite a candle. The light of the candle will shine brighter in the night than during the day. The Netziv explains that this is because during nighttime the creation of darkness prevails, making the light of the candle burn brighter.
It seems that even those who hold that darkness is generally nothing more than the absence of light agree that the makkah of choshech in Mitzrayim was a creation of darkness. This is what the Torah describes as a tangible darkness. Those opining that darkness is a creation, note that the darkness of makkas choshech was a higher intensity of darkness than is normally present.
It turns out that the neis of the makkah of choshech was twofold. First, the darkness was allowed to prevail even during the daytime. And second, the darkness was allowed to prevail over the light. Generally, even the creation of darkness must bend for the passage of light. The Ramban explains that during makkas choshech the thick tangible darkness even extinguished any candles that were lit.
