To Blood Seal weapons or armor, you first need to unlock Blood Seal slots. You unlock these slots by participating in combat against mobs. Only one slot on a weapon or armor piece can be unlocked this way at any one time. There does not seem to be a set amount of time for the Blood Seal ability to unlock, and sometimes it can be completely random, though they do seem to unlock faster when fighting orange or red-named mobs.

You can unlock a total of five Blood Seal slots on weapons and three on each piece of armor. This means that you will be able to have five Blood Seals on a weapon and three on armor pieces.

When a piece of armor or weapon gains a new Blood Seal slot or "ability" (as it is commonly referred to), you will get a message in red text on the Game's System Text window. The text will tell you what item has gained the Blood Seal ability.

When a Blood Seal slot has been unlocked, the item will have a red text in its description. It will remain there until a Blood Seal Gem is added to that slot. While the item has the red blood seal text, you cannot unlock another Blood Seal slot on that item until you add a Blood Seal Gem.