Blinded is a -2 to AC but Invisible grants a +2 to attack rolls. Do they stack?
Yes, two different conditions.
No. Because it is silly.
Look at it this way: If I am blind, I am immune to invisibility... because I am already blind.
Blinded is a -2 to AC but Invisible grants a +2 to attack rolls. Do they stack?
Sort of.
Blinded: "The creature cannot see. It takes a –2 penalty to Armor Class, loses its Dexterity bonus to AC (if any), and takes a –4 penalty on most Strength- and Dexterity-based skill checks and on opposed Perception skill checks. All checks and activities that rely on vision (such as reading and Perception checks based on sight) automatically fail."
Invisible: "Invisible creatures are visually undetectable. An invisible creature gains a +2 bonus on attack rolls against sighted opponents, and ignores its opponents' Dexterity bonuses to AC (if any)."
Both conditions apply, but invisible only grants the +2 bonus on attack rolls against sighted opponents, and someone who is blinded cannot see.