I did essentially the same thing. I have this input in a form:

    <label for="gb-email" class="nah" aria-hidden="true">Email:</label>
    <input id="gb-email"
           name="email"
           size="40"
           class="nah"
           tabindex="-1"
           aria-hidden="true"
           autocomplete="off"
    >

With this CSS:

    .nah {
      opacity: 0;
      position: absolute;
      top: 0;
      left: 0;
      height: 0;
      width: 0;
      z-index: -1;
    }

And any form submission with a value set for the email is blocked. It stopped 100% of the spam I was getting.