Minecraft Command Block Remote Volume Selection

You could summon the sheep with something like a custom name or scoreboard tag, then specify that name/tag in your selector:

summon Sheep ~ ~ ~ {Tags:["Summoned"]}

Then:

execute @r[type=Sheep,tag=Summoned] ~ ~ ~ setblock ~ ~-2 ~ redstone_block

There are a few ways to detect a certain block in an area:

If you just want to detect whether or not a certain block is inside an area and if you don't mind having the block destroyed, you can use this command:

/execute @a ~ ~ ~ fill ~-10 ~-10 ~-10 ~10 ~10 ~10 air 0 replace wool 14

You can detect the block placed using a comparator or a conditional chain command block.

If you, however, want to know where the block is placed and or keep the block, you can use this more complex approach.

/execute @a ~ ~ ~ fill ~-8 ~-8 ~-8 ~8 ~8 ~8 structure_void 0 keep

#Repeat this section for how many types of blocks you want to test for
/execute @a ~ ~ ~ fill ~-8 ~-8 ~-8 ~8 ~8 ~8 air 0 replace wool 11
/execute @a ~ ~ ~ fill ~-8 ~-8 ~-8 ~8 ~8 ~8 command_block 0 keep {auto:1,Command:"summon armor_stand ~ ~ ~ {Invisible:1,NoGravity:1,Marker:1,Tags:["wool11"]}"}

/execute @a ~ ~ ~ fill ~-8 ~-8 ~-8 ~8 ~8 ~8 air 0 replace wool 14
/execute @a ~ ~ ~ fill ~-8 ~-8 ~-8 ~8 ~8 ~8 command_block 0 keep {auto:1,Command:"summon armor_stand ~ ~ ~ {Invisible:1,NoGravity:1,Marker:1,Tags:["wool14"]}"}

...

/execute @a ~ ~ ~ fill ~-8 ~-8 ~-8 ~8 ~8 ~8 air 0 replace structure_void
/blockdata ~2 ~ ~ {auto:1}
/blockdata ~1 ~ ~ {auto:0}
[Impulse] /execute @a ~ ~ ~ fill ~-10 ~-10 ~-10 ~10 ~10 ~10 air replace command_block
/execute @e[type=armor_stand,tag=wool11] ~ ~ ~ say You placed blue wool!
/execute @e[type=armor_stand,tag=wool14] ~ ~ ~ say You placed red wool!