I had the same question. Initially I thought it was processed video because of the blown out highlights and it was too grainy but actually it was shot on 16mm reversal film stock, the same stuff I thought was crap that they gave us in film school in the 80's - but it just goes to show you that if you have vision/talent you can make anything work for you... or if you "position" it as "a look you're going for" because if this was given to a distributor they would say it looks like crap (and it does) and hand the reel back to you. It's just a sh-t blow-up of crap black and white 16mm reversal film stock, blown up to 35mm... bleh. But it works for this film, "I guess". It certainly is different looking...
As for the film - I have mixed feeling about how it looks. I think it "too much" and dominates the film - not in a good way. Aronofsky seems to make the same film over and over again with the same theme of obsession leading to death and tragedy - it's getting "a little" old methinks but I'm always entertained by his films. In the case of Pi, it was the quest for "Pi"... But when one realizes what Pi is - the ratio of the radius of a circle to its circumference, which happens to be an irrational number... the proceedings happen to come off (to me anyway) more than a little silly. There's nothing mystical about it. It's just a ratio. The gates of hell are not going to open over it, and as an irrational number like any other irrational number there is no "end" to the decimal places, and any to the quest to calculate Pi to the Nth place is ridiculous. It's nothing to go insane over. It's just a ratio, like 2/3rds...
Anyway... 16mm reversal film stock, blown up to 35 to make it look even crappie...errr... more "aesthetic".