What are the possible causes of recurrent hypersomnia? Ie sleeping more then 8 hrs per night[up to 10 to 12 hrs and possible cure, blood tests were ok

Sleepiness. Please do a sleep study to rule sleep apnea or sleep related disorders also rule out depresssion, medical causes, sleep apnea etc.
OSA? Consult with a sleep physician. You could have a sleep disorder that would be uncovered in a sleep study.