Polypropylene mix

[joeprocess]

Replace the black with whatever % concentrate required and up the percentage of HDPE to cover the rest. If the material is pre-colored I would go with a 93%/7% mix.

[chrisprocess]

Living hinges are tricky when you're dealing with fit form function of mating components in an assembly or whatnot.

Typically thickness of the hinge determines the stiffness, though 250um is already on the thin side. (thin enough it may also be contributing to your sink areas you experience.)

Does the hinge span across the entire length of the part? Can you reduce the length of the hinge to help stiffness?
The hinge design itself has a lot to do as well.. Really you want the hinge to be able to swivel in two separate areas not just one. Like if you just had a sharp "triangle" type feature cut into a small strip of plastic it just won't swivel properly because you'd be stretching material from either side of the strip of plastic. If that makes any sense.. Also anything you can do to soften up rough edges will help prevent shear (brittleness and lack of strength) and encourage polymer orientation for improved strength and longevity.

Also it helps a lot of actuate the hinge while it's still hot and then letting it cool afterwards
As for material I wouldn't know the best recommendation I'd imagine any PP would probably work? Definitely consult material expert but maybe you can play with HDPE + PP mix ratios to gt your desired stiffness in the mean time..