Blue items always give something and always take something. Bought with Lunar Coins, which carry over between runs.
Beads of Fealty
Seems to do nothing... but it opens a secret path in the game.
Watch out: The safest lunar to trade at a Cleansing Pool.

Brittle Crown
Your hits sometimes give you gold, but taking damage costs you gold. Each extra crown, more both ways.
Stacking: Each copy adds the same, no cap.

Corpsebloom
You heal double, but slowly and with a per-second cap. Each extra one heals more but lowers the cap.
Stacking: Healing adds the same; the per-second cap drops with each copy.
Watch out: It stores at most your full health in pending healing.

Defiant Gouge
Using a shrine spawns enemies nearby. Harder, but more experience and gold.
Stacking: Each copy brings stronger or more enemies (and more gold).
Watch out: At very high difficulty it stops working.

Egocentrism
You keep getting bombs that orbit you and explode on enemies, but every 60 s it eats one of your items and turns it into this.
Stacking: Each copy spawns orbs faster and adds one to the cap.
Watch out: It eats any item at random, even ones you have 30 of.

Essence of Heresy
Replaces your special skill: your hits mark enemies and using it blows up every mark at once, wherever they are.
Stacking: Each copy adds the same, no cap.
Needs unlocking first (challenge «Blockade Breaker»).

Eulogy Zero
Items sometimes (5%) turn out lunar instead of normal.
Stacking: Each copy adds the same, no cap.
Watch out: It affects the whole team and only kicks in next stage.

Focused Convergence
The teleporter charges 30% faster, but the area you need to stand in is half as big.
Stacking: Capped at 3: more do nothing.
Watch out: Half the radius is a quarter of the floor area: worse than it sounds.
Needs unlocking first (challenge «Never Back Down»).

Gesture of the Drowned
Your equipment recharges in half the time, but fires on its own as soon as it is ready, whether you want it or not.
Stacking: Each copy cuts a share of what is left: it drops less each time and never reaches zero.
Needs unlocking first (challenge «The Demons And The Crabs»).

Hooks of Heresy
Replaces your secondary skill with a ball that hurts what is around it and explodes after 3 s, rooting enemies.
Stacking: Each copy adds the same, no cap.
Needs unlocking first (challenge «Blockade Breaker»).

Light Flux Pauldron
Your skills come back twice as fast, but you attack at half speed.
Stacking: Cooldowns halve each time; attack speed drops less each time and stops dropping at 9.

Longstanding Solitude
All your gold turns into experience; on level up, your next purchase is free. Everything costs 50% more.
Stacking: It has caps: past 3 copies, and past 8, some parts stop growing.

Mercurial Rachis
Creates an area nearby that gives 50% more damage to everything inside, you and enemies alike.
Stacking: The area grows faster each time: each copy adds 50% of the previous radius.
Watch out: Enemies get in too.

Neutronium Weight
Makes you worse at everything in combat, but you pass it to an enemy when you hit them. A curse to hand off.
Stacking: Each copy makes it worse.
Watch out: It goes to the nearest character on the other team, not the killer. Leave the stage while it is in flight and it is gone.

Purity
Your skills come back 2 s sooner, but luck turns against you: everything random rolls worse.
Stacking: Each copy adds the same, no cap.
Watch out: Cooldowns never go below half a second. Each Purity cancels a Clover. Does not affect Tougher Times.
Needs unlocking first (challenge «The Calm»).

Shaped Glass
You do double damage but have half the health. Each extra glass doubles damage and halves health again.
Stacking: Multiplies: 2, 4, 8 times the damage, and health halves each time.
Watch out: It removes one-shot protection. At 25 copies you die instantly (game limit).

Stone Flux Pauldron
Double health, but you move at half speed.
Stacking: Health adds the same; speed is divided by 1 plus your copies: the second takes less.

Strides of Heresy
Replaces your movement skill: you become untouchable for 3 s, run faster and heal 25%. Each extra one, longer and more healing.
Stacking: Each copy adds the same, no cap.
Watch out: It does not clear burns, nor save you from Shrines of Blood or falls.
Needs unlocking first (challenge «Blockade Breaker»).

Transcendence
All your health becomes shield, which refills if you avoid hits for a while, and you get 50% more. Each extra one, 25% more.
Stacking: Each copy adds the same, no cap.
Watch out: Healing items become nearly useless. Damage over time delays shield regeneration.

Visions of Heresy
Replaces your main attack with a flurry of homing shards that explode. Each extra one, more charges.
Stacking: Each copy adds the same, no cap.
Needs unlocking first (challenge «Blockade Breaker»).

With Lunar Coins: 1 per Lunar Pod on the stages (random item) and 2 per Lunar Bud in the Bazaar Between Time, where you see the item before buying.
Enemies have a 0.5% chance to drop one, halved each time one drops in the run. Also 10 for beating the game, 10 for escaping the Planetarium, 5 for obliterating at the Obelisk and 1 to 10 per challenge. They carry over between runs.
Yes: a Cleansing Pool trades a random lunar item for a Pearl (80%) or an Irradiant Pearl (20%).