What is the ruling on performing ghusl for janabah and prayer when there are unnoticed adhesive residues preventing water from reaching the skin, and must the prayers be repeated? Also, what is the ruling on prayer and ghusl when there is a remaining black residue that has not been removed?

The ritual bath (ghusl (definition)) is not valid if there is an obstruction preventing water from reaching the skin, according to the majority of scholars, unless the obstruction is minor, in which case it is excused by Sheikh al-Islam. It is more prudent to invalidate the prayers performed with such an obstruction and to re-perform any prayers where you were unable to remove the obstruction by wiping over it with water.