It is somewhat blue, not like Jango blue, but a very faint colour. I personally chose grey, it's more Fett to me. I'm an ESB Fett with ROTJ flight suit, and the weathering on my armor is ROTJ. As long as it's Fett, I choose what looks best
Here are some publicity photos shots of the ESB jumpsuit:
Though it is always hard to trust photos, you can see it is fairly blue with some grey in it..I'd say more of the former than the latter.
If you check the exhibit photos on the Ref CD, especially those from the MOM exhibit, you can see some faded blue in some of the covered areas of the jumpsuit, such as the shoulder area:
This suggests that this jumpsuit could have been one of the ones made for ESB, which was subsequently used in ROTJ. I don't know for sure, but it would make sense. As I've even found with my own jumpsuit, the sun will fade the exposed areas but underneath the vest it remains the original colour. That means that blue colour you see is probably close to how it looked in ESB.
For mine, I started with a white jumpsuit, dyed in some light blue and very little light grey:
The pre-pro suits were a darker blue color it looks like the had one color of jumpsuit that was badly dyed and it just faded over the time between pre-pro to esb (due to washing and sunlight and what not) and then even more for ROTJ.