Transformed Darkness

The Abarbanel points out that the last three makot all were forms of darkness, the makah of חשך is obvious, the locust blackened the earth and maakat bechorot took place at night.  Why is it significant that all the final maakot took place at night?  In addition, the actual leaving of Egypt took place at night, yet it was light like day (see Tehillim 139:12 and Zohar Bo 38a,) why?

Light is when things are clear; a person can see and has confidence.  Darkness is the time of mystery, lack of vision and fear.  The simple understanding would be that the golus is darkness and the geulah is light.  The message of the preparation for the geulah is that the darkness is also part of the geulah.  The message of לילה כיום יאיר, the darkness lighting up is that the darkness is transformed into light.  The maakot that culminate into the geulah process take place under the veil of darkness to demonstrate that the darkness leads to the geulah.  It is not the peshat that there is golus shrouded in darkness and then there is the light of geulah and they are two distinct existences, rather the golus is a component of the geulah.  לא אמרתי אלא בשעה שיש צה ומרור לפניך.  The marror is also part of the story of the geulah. [From an essay of Rav Yehoshua Shapiro.]