Suppose Player 1 has a Dragon on the table and Unsummon in hand, while Player 2 has two Spiders on the table.
Player 1 chooses to attack with the Dragon and Player 2 chooses to block with one of the Spiders. Before combat damage is to be dealt, Player 1 uses Unsummon on the blocking Spider.
Does Player 2 take any life damage from the attacking Dragon?
No
The player takes no damage as the Dragon remains blocked. From the Comprehensive Rules for the Declare Blockers step:
509.1h An attacking creature with one or more creatures declared as blockers for it becomes a blocked creature; one with no creatures declared as blockers for it becomes an unblocked creature. This remains unchanged until the creature is removed from combat, an effect says that it becomes blocked or unblocked, or the combat phase ends, whichever comes first. A creature remains blocked even if all the creatures blocking it are removed from combat.
An Exception is, when the attacking creature has trample. See the Comprehensive Rules for Keyword Abilities:
702.19c If an attacking creature with trample is blocked, but there are no creatures blocking it when damage is assigned, all its damage is assigned to the player or planeswalker it's attacking.
The defender doesn't take damage unless the attacking creature has trample. This is because the attacking creature is still "blocked" and so must assign its combat damage to its blockers, of which there are none. The relevant rules are as follows:
509.1h An attacking creature with one or more creatures declared as blockers for it becomes a blocked creature; one with no creatures declared as blockers for it becomes an unblocked creature. This remains unchanged until the creature is removed from combat, an effect says that it becomes blocked or unblocked, or the combat phase ends, whichever comes first. A creature remains blocked even if all the creatures blocking it are removed from combat.
510.1c A blocked creature assigns its combat damage to the creatures blocking it. If no creatures are currently blocking it (if, for example, they were destroyed or removed from combat), it assigns no combat damage.
702.19c If an attacking creature with trample is blocked, but there are no creatures blocking it when damage is assigned, all its damage is assigned to the player or planeswalker it's attacking.
In short. Whenever a creature is declared blocked (by declaring a blocking creature), damage will be done to the creature, and not to the player. If something happens (e.g. bounce, sacrifice), the damage is still directed to the creature (which is gone, therefore the damage goes nowhere (creature blocked)).
If the attacking creature has trample, it's a little different. Trample says that all damage more than toughness of the defending creature, continues to the player. Note that with trample, you have to do minimal lethal damage to blocking crature (more is allowed).
lifelink and other abilities that care about damage—which are quite common.
Commented
Mar 12, 2015 at 22:00
You wouldn't do damage to the player unless the creature has trample. Abilities like lifelink and double strike still apply. Lifelink requires damage to be done. Doublestrike mearly doubles the damage done. If no damage occurs, neither ability activates. However, if the blocking creature is removed after the damage phase, damage is still done to the blocking creature, although this will remove the damage done and the creature can survive lethal damage. If it's removed before damage can take place, due to a fast effect or instant, no damage is done to the blocker and the attacker is still blocked.
