The software uses your weight (input in your profile), power and cadence in conjunction with the % grade slope to work out your speed. When it works out that your speed should be higher/lower than the actual wheel speed it shows a speed in blue.
I suffer from this, because of my weight, in a big way. The max resistance the trainer can apply has problems simulating slope when the weight is >80kg and/or >10% slope. It is still very hard to turn over the peddles so you get a good workout.