The Corpsebloom is a new item in Risk of Rain 2.
Corpsebloom increases the incoming healing, but unlike Rejuvenation Rack not incoming regeneration. It then applies the heals over time, ticking every 0.2 seconds, at a maximum rate of 10% your max health (this maximum rate is reduced by 50% per stack). This means that a single Corpsebloom heals for 2% of your health per tick.
Rejuvenation Rack will raise the maximum healing over time received by Corpsebloom above the cap according to its multiplier.