However, unless I am mistaken, AbstractSocketAppender uses instances of ArrayBlockingQueue. SynchronousQueue are only used for queue sizes of less than 0, which are not supported except in unit tests.

From AbstractSocketAppender:

  public static final int DEFAULT_QUEUE_SIZE = 0;

  [...]

  private int queueSize = DEFAULT_QUEUE_SIZE;

  [...]

  BlockingQueue<E> newBlockingQueue(int queueSize) {
    return queueSize <= 0 ? 
        new SynchronousQueue<E>() : new ArrayBlockingQueue<E>(queueSize);
  }

Note that a SynchronousQueue is used if the queue size is 0, which it is by default.