Hi Linh,
This is a normal plate,
the black area you have measured is some synovial fluid between the plantar plate and the hyaline cartilage of the metatarsal head.
The plantar plate IS the joint capsule so fluid immediately under it is joint fluid.
The location of your fluid is not the location where a plantar plate likes to tear.
Plantar plates tear from the base of the proximal phalanx at the lateral aspect of the plate.
It is a predictable pattern.
The key findings are that the base of the proximal Px enthesis becomes irregular when the plate is torn from it. There will be a defect in the adjacent plate, and also when you stress test it there will be a disconnect between the base of the Px and the plate.
Read the plantar plate article in the “journal articles” section of the website and it will help you to understand plate tears.
Steve