when item (commandblock) is given by sign it's not visible unless you update the items slot (right click or q).
used: /setblock 3 56 2 standing_sign 5 replace {Text2:"{text:\"[CommandBlock]\",color:dark_blue,clickEvent:{action:run_command,value:\"/give @p command_block 1 0\"}}"}

In picture: There's invisible commandblock in slot 0 (first hotbar slot)