Who controls Darksteel Mutation?

Darksteel mutation is your enchantment. The creature it's enchanting is still owned and controlled by your opponent.

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One may also ask can 2 creatures block 1 attacker?

Each creature can only block a single attacker, but the defending player may choose to block an attacking creature with more than one creature. Both players are given a chance to cast instants and activate abilities after blockers have been declared. Moreover, do board wipes work on indestructible? Most board wipes "destroy" creatures, so one way to protect your creatures (and permanents) is to buff them with the "indestructible"keyword.

Does sacrifice count as dying MTG?

Yes you can . Dying is a keyword that means going from the battlefield to the graveyard. This includes sacrifice. Can Sun Titan return auras? Yes you can. When the Aura comes into play because of Sun Titans trigger, you still have to target a creature to attach the Aura to it, you can't target permanents with shroud.

Keeping this in consideration, does gaining protection from a color remove auras?

Yes. Creatures with protection (from blue) can't be damaged, enchanted, blocked, or targeted by anything (blue). The Aura is removed and goes to the graveyard. Can an opponent enchant a creature with Hexproof? So, you cannot Cast an Aura targeting your opponent's permanent with Hexproof. However, if you put an Aura onto the Battlefield via a method other than Casting it, then you simply choose a permanent it can legally be attached to. This does not target, so you can Enchant an opponent's permanent with Hexproof.