function BlockMessage(arg, options) {
  Message.call(this, options);
  this.Block = options.Block;
  this.command = 'block';
  $.checkArgument(
    _.isUndefined(arg) || arg instanceof this.Block,
    'An instance of Block or undefined is expected'
  );
  this.block = arg;
}
@param {Block=} arg - An instance of a Block
@param {Object} options
@param {Function} options.Block - A block constructor
@extends Message
@constructor