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With the upcoming prerelease I'm currently wondering about the new protection creatures. When playing a creature which gets protection from a certain color, is that creature then able to block an attacking creature with the color it is protected from?

For example, if you play Blightbeetle, and the opponent attacks with a green creature, am I able/allowed to block with my Blightbeetle?

The reason I'm asking is, because I'm a little confused by the text. It says "can't be targeted". Is declaring the creature as a blocker considered targeting then, or does that only include other spells and stuff?

Protection from green (This creature can't be blocked, targeted, dealt damage, enchanted, or equipped by anything green.)

Declaring a creature as a blocker isn't targeting; the rules don't mention the word 'target'.

509.1a The defending player chooses which creatures they control, if any, will block. The chosen creatures must be untapped. For each of the chosen creatures, the defending player chooses one creature for it to block that’s attacking that player or a planeswalker they control.

If you want to know what counts as being targeted (a lot of it does), you'll need to read rule 114.

In fact, declaring the creature with protection as a blocker is very often the right thing to do, as the creature won't receive any combat damage and thus is likely to survive.

Your creature having protection from green does not prevent you from blocking a green creature. Attacking does not involve targeting, and in any case only requires the attacker to choose players or planeswalkers to attack, never the defending creatures.

As you noted, a creature with protection from something can't be blocked, targeted, dealt damage, enchanted, or equipped by anything that has that "something" quality. That quality is usually one or more colors.

702.16a Protection is a static ability, written “Protection from [quality].” This quality is usually a color (as in “protection from black”) but can be any characteristic value or information. [..]

As for your question, targeting can only happen when someone casts a spell, activates an activated ability, or triggers a triggered ability, as specified in 114.1 a-f. Nothing else in the game involves targeting. In particular, attacking and blocking does not involve targeting in any way, and therefore that untargetability part of protection is irrelevant.

509.1a The defending player chooses which creatures they control, if any, will block. The chosen creatures must be untapped. For each of the chosen creatures, the defending player chooses one creature for it to block that’s attacking that player or a planeswalker they control.

Taken from the MTG Wiki:

Protection is commonly misunderstood as complete exemption from permanents, and effects created by cards, with the specified quality. However, protection is defined by a relatively narrow set of rules, which are often communicated using the mnemonic acronym DEBT. The object with protection cannot be:

• 
  Damaged by sources with the specified quality. (All such damage is prevented.)


• 
  Enchanted, equipped, or fortified by permanents with the specified quality.


• 
  Blocked by creatures with the specified quality.


• 
  Targeted by spells with the specified quality, or by abilities from sources of that quality.

So your creature can block the creature it has protection from (without taking damage), but cannot be blocked by it.

As for the confusion about targetting, there is following rule (emphasis mine):

114.1. Some spells and abilities require their controller to choose one or more targets for them. The targets are object(s) and/or player(s) the spell or ability will affect. These targets are declared as part of the process of putting the spell or ability on the stack. The targets can’t be changed except by another spell or ability that explicitly says it can do so.

So the action of declaring the creature to block does not involve choosing a target, only spells and abilities that use the word "target" do.