The point is to get the light to illuminate the specimen but miss a direct hit of the objective. With the darkfield stop the light comes to the specimen at a sharp angle, away from the objective. Check this link and can scroll down to "How does darkfield illumination work":

One-Penny Darkfield Illumination Filter DIY Science and Electronics Projects

There are a couple of diagrams of the light path with and without a darkfield stop.

- Alex