A Blessed Orb is a currency item that can be used to re-roll values of the implicit modifiers on a piece of equipment. (This means the top values of an item above the line and not the bottom 6 values) This can only be used on an item that actually has implicit modifiers that can have a range of values.
Blessed Orb on Uniques
Like normal items, if the unique items have implicit modifiers, it can be rolled by Blessed Orbs. For example, the implicit mods of Eye of Chayula are “+16 To All Attributes” and may reroll to “+14 To All Attributes” by Blessed Orb.
Likewise, if the unique items are Corrupted, or without implicit modifiers, or the value of implicit mods have not ranged, Blessed Orb could not reroll it.
A Blessed Orb re-rolls the implicit modifier of an item. It cannot be used on items without implicit modifiers or whose implicit modifiers do not have a range of values. They cannot be used to alter implicit modifiers on corrupted items, because such items cannot be modified in any way. Because of their rarity and limited impact, Blessed Orbs are best used to enhance high-end equipment rather than equipment used during the leveling process.
An Exalted Orb can be exchanged for 360 Blessed Orbs. You can farm Exalted Orbs to obtain this poe currency as well.
Blessed orb can not roll the same implicit value that the original item had. If the current implicit value is low, then on average you have better odds of rolling a high implicit value. And vice versa.
For example - when using blessed orb on 9% Chaos resistance Amethyst Ring normally you would have 20% chance to roll 13% Chaos resistance. But since it is impossible to roll 9% you actually have 25% chance to get 13% resistance.