The black spot will not go away as it is the eyes of the fry through the pale skin of the guppy. As the pregnancy progressess, the (black) gravid spot will get darker, and the fry will mature.
That is quite common. Don't worry about it. A gravid spot does not have to be dark or black. Your guppy is perfectly normal.
Many male guppies have 'black spots' but if by "black spot" you mean "gravid spot" then no, a male guppy can not be 'gravid' because gravid is the equivalent of "pregnant" and male guppies can not get pregnant.
The telltale black spot or eye spot that appears on the flank of a pregnant livebreeder (swordtail, guppy, molly, platy) is actually the dark eyes of the embryos forming inside her. The black spot only disappears when the babies do, so she has either aborted (due to stress, the death of the majority of the embryos, trauma eg. being hit in the abdomen or some other unknown reason) or given birth to the brood, which are tiny and will hide in the gravel, in the plants, or on ornaments and can be very hard to see.
Your guppy will be pregnant if it has been in a container with a male since it reached maturity. A none pregnant guppy is an unusual thing. She will develop a belly and also when seen from behind she will have a dark spot visible deep inside her on either side of her vent. (the gravid spot)
The female guppy will develop a dark spot on the side of her belly during pregnancy. As the pregnancy progresses, the spot will grow darker.
A pregnant female will have a black spot near their tail that continues to grow in size as she gets closer to giving birth.
You have said she has a gravid spot (not gravel). That indicates that she is gravid (pregnant). She would not have that spot is she was not gravid. Be patient. Provided she is in suitable conditions she will have her youngsters eventually.
There will be a dark spot in front of the fishes anus (cloaca) in a pregnant (gravid) female livebearer. This spot is called the "Gravid Spot".
When your female guppy is pregnant you can't tell by it's size. You have to look at it's gravid spot. Which is right behind their stomach and the tail. It's right around the butt of the guppy. It takes about a month for the babies to be born. Plus you can not stress out your the pregnant fish.
The gravid spot can be seen from both sides of a female guppy, but it is located directly in front of the anal fin on the bottom of their abdomen.
The gravid spot on a guppy is not always a 100% determination of pregnancy as the colours can vary. Quite often, when they are ready to give birth, the gravid spot becomes quite dark.
Try explaining why it's there.. it means the fish is pregnant. The darker it gets the closer she is to giving birth.. The gravid spot on a female guppy is a dark area directly in front of the anal fin.