A Bless protects part of the experience you would lose when you die. You buy it at the Ho-Oh Statue in Join Avenue; it stays active until your next death that costs experience, and is used up on that death.
The protection is a share of the experience you would lose. For example, if a death would take 1,000,000 experience:
protection
You lose 600,000
protection
You lose 400,000
protection
You lose 200,000
The Bless is applied after any other loss reduction you already have, such as the Elite Trainer one, and protects its share of what is left. For the most protection against experience loss, become an Elite Trainer: look for the NPC Pokémon Master at the Pokémon Center in Singer.
Go to the Ho-Oh Statue in Join Avenue, next to the Pokémon Center, and use it. The Choose your Bless window opens with the three tiers, and the protection and price of each for your level.
Pick a Bless and confirm the purchase.
Money in your backpack is used first; if you are not carrying enough, the whole price is taken from your bank balance (or the other way round: the whole price is taken from your backpack if the bank does not hold enough). The debit shows on the bank statement as bless, and you get a notice in chat.
You can only have one active Bless at a time, but you can swap it for a higher one whenever you like and pay only the difference.
Upgrade cost = price of the new Bless at your current level − what you already paid for the active Bless
No downgrade: with a Bless active, you cannot buy the same tier or a lower one.
No refund: a Bless cannot be sold or cancelled.
The Bless is used up on your next death that takes experience. On that death, a chat message tells you how much it protected.
Deaths that take no experience do not spend the Bless. Dying in a PvP area, for example, keeps it active for next time.
It stays active after you log out and after server restarts, until it is used.
Once it is used up, the price you paid goes with it. The next purchase is at the full price.
While it is active, an icon on the interface shows the protected percentage when you hover over it.