`BLMPOP` is the blocking variant of `LMPOP`.

When any of the lists contains elements, this command behaves exactly like `LMPOP`.
When used inside a `MULTI`/`EXEC` block, this command behaves exactly like `LMPOP`.
When all lists are empty, Redis will block the connection until another client pushes to it or until the `timeout` (a double value specifying the maximum number of seconds to block) elapses.
A `timeout` of zero can be used to block indefinitely.

See `LMPOP` for more information.

@return

@array-reply: specifically:

* A `nil` when no element could be popped, and timeout is reached.
* A two-element array with the first element being the name of the key from which elements were popped, and the second element is an array of elements.