Unable to get Azure storage key from storage resource in ARM template

As it turns out the structure of the object returned from listKeys is an array of keys which looks like

[
  { "keyName":"key1", "permissions":"Full", "value":"keyvalue1"},
  { "keyName":"key2", "permissions":"Full", "value":"keyvalue2"}
]

So the correct solution to getting the value out was to do keys[0].value.