If you have a movable light sensor, then scan the entire sample and store the values, say in an array. Let the value for black be 0. If your array has only 0's, you have a blob. If your arrays have 0 only in the start and the end, you have a circle. Not too sure about the blob in the circle, but if you have continuous 0's separated in 2 places by non zeroes, that is three 'groups' of zeroes, then maybe you have a blob in a circle.