I was recently playing Fortnite/watchingsome videos, and a lot of people are referring to 'bloom'. I have googled and found some information on it, but I am a little confused on how its implemented. Is it a completely random spot in your cursor that the bullet hits, or is there some kind of first shot accuracy?
As @Kevin stated, this can be easily confused with graphical effect Bloom without knowing the context in which the term is being used.
In the context of shooter games, the term is known as Reticle Bloom and is very similar to the term Weapon Spread. Reticle Bloom and Weapon Spread both relate to the loss of accuracy when firing, but the difference is where the inaccuracy is lost.
Weapon Spread, like this answer states, is where the bullet looses accuracy in relation to the distance of its target. A good example for this would be a Shotgun.
Reticle Bloom is where the loss of accuracy is centered on the gun and is in relation to how fast the gun is being fired, and the target Reticle becomes larger if not ceased.
Halopedia's definition for this is
reticle bloom, this is where sustained fire will decrease the accuracy of the weapon being fired and also increases the size of the reticle
Using a gif borrowed from giphy of the game Halo Reach, you can see the reticle become larger after continual, rapid shots making it more difficult to aim
Bloom is the spread of the bullets in the aimer, or the four white markers in the center. The Bloom of a gun is its random chance of hitting the target straight on. So, the more bloom a gun has in fortnite, the less chance the bullet will fly perfectly straight, no matter if you are crouching or not moving.
Bloom is the +/- of inaccuracy when moving/crouching/fully auto firing/etc.
Bloom changes the cone of fire.
Bloom is not the cone of fire or how bullets randomly land within it. If, for example, the corsshair was fixed and the cone of fire did not change the game would no longer have bloom yet bullets would still land randomly inside the cone of fire.
However the fortnitebr community has attached the idea of all shooting mechanics in fortnite to bloom. Which makes discussing it somewhat confusing.