If by a black dot you mean a black/ darker speckle, than that is a sign of a 'tight' shot. i.e. leaning towards a ristretto type shot or resticted pour. This is not a bad thing, just one stlye of extraction. The lighter spot, if it is apearing at the end of the pour is a sign of overextraction. This is a bad sign. If you are interested in looking into it further, pour 2 espresso from a double handle. Collect 1 side as you would normally for an espresso in 1 cup. In the other side use 3 cups and collect 1/3 of the espresso in each. The 3 parts of the espresso are very different. Examine the crema of each, and taste each. This will give you a better idea of what is happening in the different stages of the pour.
