There will probably be some improvement from utilizing samatha jhanas as well as improvement from reaching certain stabilized plateaus. I could definitely see intelligence decreasing during dark night or other difficul places in the path. This might not be an actual decrease in intelligence, but rather an artificial seeming decrease due to a lack of ability to concentrate, focus, apathy, and dwelling on existential topics.
Here are a few things, other than meditation, that have empirically been shown to increse intelligence:
http://brainworkshop.sourceforge.nethttp://www.thefreelibrary.com/Vitamin+and+mineral+supplements+increase+intelligence.-aa08542509