Mangekyo Blindness and Eye Strain

The Mangekyo's power costs sight. Each eye tracks its own blindness, and heavy use also causes short-term strain.

Which Eye Each Technique Uses

Right eye: Itachi's Amaterasu, Kagutsuchi, Kamui (intangibility and self-warp), Kotoamatsukami.
Left eye: Sasuke's Amaterasu, Tsukuyomi, Kamui: Warp (snap and take).
Either eye (the one that sees better takes the blindness): Tengai Shinsei, Shisui of the Body Flicker, and opening the Mangekyo itself.
Susanoo needs BOTH eyes holding that Mangekyo; otherwise it's refused with "needs both eyes".
Only Kamui (right) and Kamui: Warp (left) need the eye physically in one particular socket.
A technique whose eye is fully blind is refused ("Your left/right eye is blind"). A technique either eye can cast is refused only when both are blind.

Permanent Blindness

Each eye has its own blindness from 0% to 100% (fully blind). It is permanent.

Every Mangekyo technique dims the eye that casts it by 0.0075% per point of strain it adds. For an either-eye technique, the eye with less blindness takes it.

Tsukuyomi: 160 strain = 1.2% (about eighty casts to blind the eye).
Kamui: Warp snap: 160 strain = 1.2%. Take: 180 = 1.35%.
Kamui self-warp: 120 = 0.9%. Kamui held: 8 strain a second = 0.06% a second.
Kagutsuchi: 160 = 1.2%. Kotoamatsukami: 260 = 1.95%. Tengai Shinsei: 200 = 1.5%. Body Flicker: 60 = 0.45%.
Amaterasu held: 3 strain per burst, about 20 a second = about 0.13% a second of that eye.

Holding a Mangekyo open: each second, each eye that isn't yet blind gains 0.01%. On its own, that would blind an eye after about 10,000 seconds (about 2 hours 47 minutes) of open time.
Warnings: as an eye passes 50%, 75% and 90% blindness, a brief popup in the middle of the screen says "Your sight is fading" / "Left eye: 50% blind" (or the right eye, and 75% or 90%), with a heartbeat.
Creative mode players and NPC bodies never go blind.

At 100%, a popup in the middle of the screen says "Your left eye is blind" (or right) / "Its Mangekyo techniques are gone", with a dark-red chat message ("The light fades from your left eye. It has gone blind.") and a slow heartbeat.
That eye can no longer cast any of the Mangekyo's techniques; the other eye's techniques still work.
With both eyes blind, an open Mangekyo closes and won't open again ("blind"). Susanoo needs both eyes in their sockets, but is only refused for blindness when both are blind.
A blind eye is drawn with a clouded iris.

Restoring Sight

A new eye in the socket sees clearly. Implanting an eye from a jar resets that socket's blindness to 0%.
Taking an eye out of a socket also resets that socket's value.

The Mangekyo Sight Panel

Shown in the stats column of the Character page whenever you have a Mangekyo (your own or implanted). It has two rows, "Left eye" and "Right eye", each showing "x.x% blind" (or "Blind" in dark red) and a thin bar that fills as the eye worsens.

Eye Strain

Separate from permanent blindness, Mangekyo techniques add strain to a pool that fades by 5 points a second.

When the pool first passes 600: Darkness for 2 seconds, once.
When the pool first passes 900: Blindness for 2 seconds, once. Strain never blinds you for longer than that: lasting blindness only comes when an eye reaches 100%.
Overuse warnings (a brief popup in the middle of the screen): when the pool passes 600, "Your eyes are straining" / "Ease off the Mangekyo: your sight is darkening"; when it passes 900, "Mangekyo overuse!" / "Stop now: your eyes are going dark". Not in creative mode.

Spamming Mangekyo techniques briefly darkens and then blinds your view until the strain fades.

Blood Tears

Each eye also keeps its own count of recent strain. Push one eye too hard and it bleeds.

Every Mangekyo technique adds its strain to the eye that cast it (for an either-eye technique, the eye that sees better). While a Susanoo is out, both eyes gain 28 a second.
Each eye's count fades by 5 a second, and never climbs past 750.
When an eye's count passes 450, it starts to bleed, with a dark-red message: "Your left eye bleeds: you've pushed the Mangekyo too far." (or the right), or "Both your eyes bleed: Susanoo is tearing at them." It keeps bleeding until its count falls back under 150.
In practice: about three Tsukuyomi in a row bleeds the left eye, and holding a Susanoo out bleeds both eyes after about 20 seconds. Once the overuse stops, the bleeding ends within about two minutes.
The look: a bright-red streak of blood runs from under the bleeding eye down the cheek toward the jaw, with a short drip on the nose side. While that eye's Mangekyo is open its white is also bloodshot with red veins. Everyone around you sees it.
The bleeding is a warning, not a wound: it does no damage and doesn't blind you by itself. It also shows in creative mode.