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Blink is a non-concentration spell, so you can easily have both up. Does anything prevent you from using blink, then Polymorph and continue to blink?
The blink effect isn't a class feature like rage, so it shouldn't be cancelled when you polymorph yourself.
I see no issue doing this myself as it takes two turns to setup.
First of all, excellent idea. One of the main impediments to polymorphing yourself into something dangerous is maintaining concentration as enemies repeatedly hit you. This strategy would often make that much more viable.
I did want to give you one word of warning though. Blink has the following rules on reappearing (PHB, p. 219):
At the start of your next turn, and when the spell ends if you are on the Ethereal Plane, you return to an unoccupied space of your choice that you can see within 10 feet of the space you vanished from. If no unoccupied space is available within that range, you appear in the nearest unoccupied space.
Keep in mind that a T-Rex is a Huge size creature, so it will require a 15 foot by 15 foot square of "unoccupied space" to reappear. In some environments (especially those outdoors), this will be easily accomplished. But in certain dungeons, it might be more difficult.
While you are on the Etherial Plane, your enemies (and allies) could move around the battle field. When your turn comes again, it's possible that every 15' square is occupied by at least one other creature. If this happens, you could be shunted to another nearby space, such as a second room. And if that room doesn't have an exit which is 10 feet wide or wider, you won't be able to leave (unless you dropped concentration on Polymorph)!
Other than these considerations, this plan is excellent. Have fun!
