Permanents with indestructible means that a permanent cannot be destroyed by effects that would destroy it or by receiving lethal damage. However that does not mean that indestructible permanents cannot be targeted by spells and abilities that would attempt to destroy them.

For example, you can use Hoard-Smelter Dragon‘s activated ability and target a Blightsteel Colossus. When this ability resolves, the Blightsteel Colossus will not be destroyed. However the Dragon will get +12/+0 until the end of turn.

Similarly, you can target a Darksteel Myr with Slice in Twain. The Myr will not be destroyed, but the you will end up drawing a card when the Slice in Twain resolves.