Using Blood Seal Gems on Armor or Weapons
When you have unlocked a Blood Seal slot and you want to add a Blood Seal Gem, it works the same way as upgrading items with stones. CLick the Blood Seal Gem icon in your inventory and drag it to the equipped item with a Blood Seal ability. You will be asked to confirm, and then the random bonus from the Blood Seal Gem will be applied. You cannot fail to Blood Seal an item. It will always be a successful process. Though it is possible to sometimes get a Blood Seal bonus that is entirely useless to your character class.

Removing Blood Seals
It is possible to remove Blood Seals from an item. This can be incredibly useful when you have had a bonus added that is completely useless to your character. To remove Blood Seal bonuses, you will need a Purification Gem. These can be purchased from a merchant in most towns for 3,000 Gold.

You can only use a Purification Gem when all Blood Seal slots have been unlocked on the item and Blood Seals added. You cannot remove one Blood Seal without removing the others at the same time. To use a Purification Gem, click the Gem's icon in your inventory and drag to the item you wish to purge of Blood Seals. Clicking confirm will remove the seals and return the item to its original state. The downside to this process is that you will have to unlock the Blood Seal slots for that item all over again. But at higher levels, it can be far more helpful to try for perfect bonuses rather than have a useless one attributed to the item.