Block

Block is an action of defending against enemy attacks. It is performed by holding two fingers on the screen. No other actions can be taken while blocking.

Mechanism

Block reduces damage taken from Basic attacks by 25% and from Special Attacks by 50%, while also greatly reduces crit chance. Unblockable attacks, such as Super Moves, cannot be blocked. Blocking attacks will not generate any power, other than from some characters' passive such as Nightwing/Batman Ninja, Wonder Woman/Regime, Static, and Superman/Godfall.

Blocked attacks can't crit, except DOT. On player's control, Deathstroke/Flashpoint's assists on specials can crit against blocked opponents. Harley Quinn/Suicide Squad's Joker attack on SP2 will ignore block (it's not unblockable, it just deals full damage and can crit regardless of block).

Block will negate some negative Status effects such as stun or freeze, but it has no effect on damage over time (although they used to prevent them from being applied, it is no longer the case), slow, snare and only reduces power drain. Disable special from passives is not affected. However, gear on-special power drains and disable specials cannot affect a blocking opponent.

AI block

Blocking works slightly different for the AI:

• The AI can "bounce up" and block while knocked down.
• The AI's block prevents you from using more than 1 hit of a basic attack combo. The Flash, Killer Croc/Arkham and some Batgirl characters are exceptional, as their heavy combos hit twice in the first swipe (Killer Croc can swipe twice unlike the other). Scorpion/Klassic is the only one that can use full heavy combo against a blocking AI enemy.
• The AI can block if you use a special attack while The Master's Death Cart's tag in damage is firing and they did not block it, which is normally not possible for the player. They can do this even if they are stunned beforehand.
• The AI can still block The Flash/Metahuman's basic combos when his passive is active, even after the first hit lands unblocked or breaks block.
• Batgirl/Arkham Knight's passive grants her a chance to break block on every hit of her specials - "block breaker" may display on the AI opponent while not actually breaking their block.

Stacking

Block gear stacks additively with block. For example, against a basic attack, if you have 17% further reduction from block gear, you will take 100 - 25% - 17% = 58% damage. Block and block gear's effectiveness is capped at 90% total, or a minimum of 10% damage taken. Against extremely weak opponents, this can actually reduce damage under 1 and may produce hits that don't do damage.

Superman/Godfall reflects 50% basic damage when blocking. This stacks multiplicatively with block and block gear. For example, when blocking with maxed LexCorp Helmet V2 and Charged Disc, he takes (100% - 25% - 40% - 22%) * 50% = 6.5% damage. If block gear reached its cap (only 10% damage taken), basic attacks will only deal 5% damage to Superman while blocking.

Basic combo interruption

The AI's block is different from players blocking in that a blocking AI will interrupt your basic attack combo (i.e. you can only hit them once, then you have to start your basic combo again) while player block does not have this effect. This is mostly advantageous to the AI, as they can do more basic hits and finish combos, particularly noticeable for combo ender effects like Martian Manhunter/Prime's passive or Ra's Al Ghul's Scimitar. On the other hand, for example, The Flash has a very fast heavy attack animation, it hits twice with the first hit but has a long finishing animation; this means players can stop attacking and block faster than the AI can while spamming basics against a blocking opponent. Batgirl/Cassandra Cain and Batgirl/Arkham Knight have their heavy combo function similar to Flash. Killer Croc/Arkham can execute the first 2 swipe of his heavy combo, and Scorpion/Klassic can even perform a full heavy combo against a blocking AI enemy.

Avoiding block

The obvious way is to use unblockable attacks like Super Moves, but there are other ways to get Special Attacks in without them being blocked.

1. Stun or freeze them beforehand, which also requires the stun or freeze to not be blocked or be unblockable in the first place, although this is easier with Martian Manhunter or Green Arrow/Arrow since their passives grant their light basic combos a chance to stun/freeze, or Bane/Luchador who has a chance to stun on tag in.
2. Block enemy basic attacks, release block and use a special in between their basic attacks. This relies on quick reflexes and usually slow enemy attack speed.
3. Use a special while being hit by enemy basic attacks. Not all basic attack patterns can be exploited like this - the ones that can are sometimes referred to as "vulnerable". Almost no light basic attacks are vulnerable, while many heavy basic attack patterns are vulnerable (a well-known example being Solomon Grundy). Darkseid is the only character known to have both basic attack patterns vulnerable, making it easy to land unblocked specials on him. The Arkham Knight/Arkham Knight is unique in that his 3-hit Combo ender is vulnerable. Note that slow attack speeds does not always equal vulnerable combos: Bane has a slow attack speed but neither of his basic attack combos are vulnerable. However, method 2 does apply for him - the second and third hit of his light basic attack combo for instance have a large delay between them, making it easy to use method 2 against it.
4. Use a special immediately after your basic attacks, referred as "Basic-Special combos". This is only available to a handful of characters, the most well known of which is Wonder Woman's heavy basic attack combo into a Shield Toss.
5. Knock down your opponent, and then when they get up and try to use basic attacks on you, immediately activate the special. Some characters, such as Solomon Grundy, has attack speeds so slow it is viable simply just standing next to him and tap special when he starts his basic attack animation. This is the most universal way to use specials unblocked.

The Master's Death Cart can also reduce enemy block's effectiveness on tag-in.

Trivia

• Batman/Arkham Knight, Superman/Injustice 2, and The Joker Unhinged/Suicide Squad have part of their SP2's animations changed when used on an enemy that is blocking. Darkseid/Prime, Wonder Woman/Dawn of Justice, Batman/Flashpoint, Black Adam/New 52, The Batman Who Laughs, and The Flash/Blackest Night have hits on their SP2 that will not be performed at all if blocked, further reducing damage taken.
• For Bonus Missions that require you to not block during a match, it only registers if you block an actual hit. If you use block, but release it before actually blocking any hit, you can still complete that mission.