Tag not displayig

CanPlaceOn is an array.
Use

/give @p minecraft:obsidian 1 0 {CanPlaceOn:[minecraft:bedrock]}

Kumassa one question...
This will detect dropper with a obsidian (With Tag CanPlaceOn) only

/testforblock -73 70 -135 minecraft:dropper 8 {Items:[0:{id:minecraft:obsidian,Count:1b,Slot:0b,tag:{CanPlaceOn:[minecraft:bedrock]}}]}

This will detect dropper with a obsidian (With Tag CanPlaceOn Or Not)

/testforblock -73 70 -135 minecraft:dropper 8 {Items:[0:{id:minecraft:obsidian,Count:1b,Slot:0b}]}

Can you test for a obsidian that don't have the tag only (That if you put one with the Tag it will fail to test) ?

Sorry, no idea.