Why did the Jews not Leave during the Plague of Darkness they Egyptians where standing still and could not see them Leave?
1 - The time of redemption has not yet arrived.
2 - It was a much bigger miracle to leave when the Egyptians were watching, and could not stop them.
3 - The promise to Avraham Avinu of leaving with a Rechush Gadol had not been fulfilled yet.
4 - They were busy burying the Jews that died.
They weren't told to leave then. In fact, according to the Sanhedrin 92b, some Jews (from Sheivet Ephraim) did leave early and then were killed by the Pelishtim on the way. (see also Targum Yonasan Shemos 13:17 - that the reason why Hashem did not lead the Bnei Yisrael derech Eretz Pelishtim - the land of the Pelishtim was because they massacred the Bnei Ephraim and would see the bones and flee back to Egypt).