Their content is a secret and not revealed until purchased.
They range in Rarity from Common to Very Rare.
Once purchased, you'll receive a random Unlockable as a reward whose rarity can range from Common to Ultra Rare. Perks cannot be obtained from a Mystery Box.
Mystery Boxes used to have a chance to spawn in every Bloodweb Level. Now they will spawn at Levels 3 & 5 and then in every fifth Bloodweb.