Hisense 50h5g model the backlights are blinking the pictures is really dark but sound quality is still good what may be the issue

First I want to say I do not have the best writing skills so please look past the lack of punctuation and possible spelling errors with that said I'll give my answer .

I have seen and repaired this problem manny times in manny different brands of tvs including most if not all the so called reputable brands, and I don't recall it ever being any of the boards as mentioned in the answers above. For tv's that use led 's for the back lights I have found that the back lights will fail to turn on at all or flash on once briefly when the Set is powered on. I have found most cases this is caused by a single led that has burnt on one of the led strips inside the TV . The led backlighting basically works the same as Christmas lights do any time a single light burns out the whole stran goes out and you have to find and replace the bad light to fix the issue you can replace a single led but it requires some advanced soldering skills . You can order a new led strip and replace the bad strip locating the bad strip you need a digital volt meeter and look up how to test led with a volt meeter . I hope this answer will help others that experience this problem

Replace the TV, fixing it is very expensive and then shortly after another problem will occur. It soon becomes a money pit. The backlight and driver board is about $400.00 just for parts.

Perhaps the backlight LED driver board needs to re-seated.
Also check flex wire connectors for loose connection.
reseat or replace