An aviator harness is the best for any type of bird. Very safe and has less pressure points, only human error will allow a fly away or injury so be aware of wear and tear(I've had mine for almost a year and still works great) and never let go of the harness while outdoors.
Mitred conures are slightly larger than blue crowns (well, up to twice as big!)
Here's a photo of Charlie in a small red Feather Tether
A photo of him in a medium green Feather Tether.
And one of him in a green Aviator Harness
The Aviator is the best, but also the most expensive. Personally, I found the Feather Tether to be easier to put on and take off, but it's also bulkier and has more parts to it.
I have had the aviator recommended to me for my bird as well, but have yet to try it on my BCC. He doesn't like the flightsuit very much. I got him a Large, as was recommended, but it is big for him, and maybe one reason he dislikes it!
For spring/summer i was thinking of buying him an aviator harness and hoping he learns to love it and the freedom it gives him.