Wall of Frost

Most defensive walls buy time by simply existing: a high toughness eats one attacker per turn and the attacker comes right back next combat. The block-and-tap rider rewrites that math. A creature that runs into this wall doesn't untap on its controller's next untap step, so each attack into it costs the aggressor not one turn but two: the swing that bounced off the toughness, and the turn that creature spends sitting frozen at home. Against a single large threat, that effectively neutralizes it on alternating turns, a soft lockdown that scales with how committed the opponent is to attacking through the front line. The 0/7 body is built to survive the encounter so the rider can keep firing: nothing in this color's common removal-on-a-stick range trades with seven toughness in combat, and the wall never needs to attack to do its job. The catch is that it only triggers on blocking, so it does nothing about creatures that go around it (fliers, evasion) and nothing on an empty board; it is a tax on ground combat specifically. That makes it a clean expression of a design idea blue rarely gets to use directly: combat denial that isn't a counterspell or a bounce, slowing an opponent's clock by tying up their attackers turn after turn rather than answering them once.