When a command such as "/Setblock" is used, and Minecraft:Comparator is set as the block to be placed, the following text is received: "There is no such item with name minecraft:comparator".
To reproduce:
Issue command: "/setblock ~ ~ ~ minecraft:comparator", and you should receive the text response.
It's name is "minecraft:unpowered_comparator". And the powered one is a separate block called "minecraft:powered_comparator". Here's the wiki page on block, item and entity ids/names (you have to press "show" first): http://minecraft.gamepedia.com/Data_values#Block_IDs