That permanent dark area does not look normal and could be a skin burn, like Louis mentioned. See that you mention a UV light; what type of UV bulb and wattage is it? Also, could you please answer the below questions to see if there is something else to look at? Thank you

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“Trouble in the Frog Enclosure”
The following information will be very helpful if provided when requesting assistance with either your frog or enclosure. To help with your questions, please utilize the below list and post the information in the proper forum area to get advice from FF members that keep the same frog. This will allow for little confusion and a faster more informed response.
1. Size of enclosure
w60cm x h40cm x w30cm
2. # of inhabitants - specifically other frogs and size differences
1
3. Humidity
Not 100% sure but the water is 26oC (79oF)
4. Temperature
Currently it is at 23oC
5. Water - type - for both misting and soaking dish
fresh
6. Materials used for substrate
stones and glass pebbles
7. Enclosure set up i.e. plants (live or artificial), wood, bark and other materials.
- How were things prepared prior to being put into the viv.
The tank has been divided, 1/3 dry the other 2/3 wet. I have a filter in the wet area and a fish (loach) to do cleaning. I have an artificial plant in he water. The dry area has a log and artificial vine. I have a wooden top with a vent and light
8. Main food source
Live food from the pet shop
9. Vitamins and calcium? (how often)
Calcium every now and then
10. Lighting
24w compact UV
11. What is being used to maintain the temperature of the enclosure
Currently nothing.
12. When is the last time he/she ate
Last night
13. Have you found poop lately
yes
14. A pic would be helpful including frog and enclosure (any including cell phone pic is fine)
will add later
15. How old is the frog
juvi
16. How long have you owned him/her
about 1 month
17. Is the frog wild caught or captive bred
Captive
18. Frog food- how often and if it is diverse, what other feeders are used as treats
Mostly crickets but will and other live food occasionally (meal worms, woodies, moths etc)
19. How often the frog is handled
Every few days
20. Is the enclosure kept in a high or low traffic area
high
21. Describe enclosure maintenance (water changes, cleaning, etc)
Filter cleaned every couple of weeks, water conditioner, removal of stools dead insects as required.
by Lynn(Flybyferns) and GrifTheGreat.