Cheapest priced copy (Oath of the Gatewatch): $0.35 at Card Kingdom, near mint, their price list as of .
Iona's Blessing 
Enchantment — Aura · Uncommon · Oath of the Gatewatch · #21
Enchant creature
Enchanted creature gets +2/+2, has vigilance, and can block an additional creature each combat.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
1 printing · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Oath of the Gatewatch | 21 | Uncommon | $0.35 | $0.49 |
One ruling from Wizards of the Coast
- 22 January 2016The ability to block an additional creature is cumulative. If a creature is enchanted with two Iona’s Blessings, it can block three creatures each combat. (The +2/+2 is also cumulative, but you knew that.)