[RedDevil]

Imagine a paddle boat in a narrow river.
On both sides of the river is a road.
The paddle boat has an extension with wheels on either side.
The wheels are on the road.
The wheels are geared to the paddle.
The paddle moves the water only half as fast as the wheels go over the road.

This paddleboat will go faster than the stream, just like Blackbird goes faster than the wind.
As the paddles move the water backwards compared to the boat, it does so less fast than the wheels move; so the scoops of the paddle actually move forward, even when they touch the water. Just less fast than the boat.

So the water flow in the river will push against the paddle scoops until the boat goes twice as fast as the water. It won't get just that fast if there is any friction, but it will go faster than the stream.
The boat would go as fast as the stream anyway if the wheels don't touch the road, so with the added push of the paddles for half the resistance on the wheels (as they cover twice the distance to power the paddle) it would go faster.

This is of course the same effect as the Blackbird. The boat could use a propeller instead of a paddle wheel for a more modern approach and an even closer resemblance