Simply being blue isn't sufficient. It must contain a suitable blue pigment that works for textiles.
A true, solid blue was extremely hard to get in dark age formulations. Most were either bluish purples, or were not wash or lightfast.
The gold standard for blue for a VERY long time was soda ash woad bended with raw lime. This plant contained the same pigment as the now much more comon indigo plant, indigotin. Indigotin itself is NOT blue, but is instead off yellow. It forms a very strong compound ring matrix with another indigotin molecule, forming the indigo blue dimeric molecule. The central ring structure makes it very resistant to chemical attack, which is why it is such a stable substance, and useful as a textile pigment. This structure is temporarily broken by the action of ammonia mixed with lime water, allowing the pigment to be carried into the textile fibers. As the ammonia dries, the ring structure reappears, and the molecule becomes hydrophobic. It gets trapped inside the fiber, and can't be washed out.
Indigo contains much more indigotin than does woad, which is why it eventually displaced it. Most indigo dye used to day (it is used to make blue jeans) is produced synthetically from petrochemical feedstocks, but is still the same resulting molecule.
In terms of textiles, you either use indigo blue, prussian blue, or
pthalo blue (a modern synthetic molecule produced using copper.)
This means either dimple cups contain cyanates (may explain their inedibility), maing them natural sources of prussian blue, or they are not themselves "blue", but instead produce blue dye, like indigo/woad, or they have very unique chemistry indeed, and produce blue copper pthalo.
I'd say dimple cups are simply fantastical, and don't really have a suitable realworld analog.
*edit
Some research in the less than ordinary channels turns up 3 species of mushroom theoretically capable of producing indigotin, but none of them are blue.
Agaricus campestris mutant
Morchella rotunda mutant
Schizophyllum commune mutant
The first is an ordinary yard toadstool.
The second is a morel subspecies
And the last is a very common species of tree shelf fungus.
To my knowledge, none of them can produce useful quantities of indigo blue, but it does suggest that such a creature is theoretically possible.