If you don't want to use a human standing next to the blind singer then it could be solved with some nice cheap modern technology.
If you could practically do it, attach a MEMS accelerometer (or gyro) to the baton and track the velocity of the tip (or equivalent reference point). Either wired or wirelessly transmit that velocity data to small processing box that drives a haptic device to alert the singer. Unfortunately I'm not entirely familiar with the visual clues of conducting, I've had a look at the patterns of motion on wikipedia and it seems the beat occurs at the point of zero vertical velocity following a downward stroke. If this is the case it should be relatively easy to process the velocity/acceleration information. An algorithm that has an understanding of the expected stroke would be better than a simply velocity test - it would potentially be more reliable as it will have a degree of inference, but minimizing latency may make that problematic.
In terms of haptic feedback, a sharp tap to the leg or hand would probably be better than a vibration as it has a more defined temporal position. Of course with a tap the processing algorithm has to be reliable...... alternatively a vibration could simply be engineered so that the magnitude of the vibration corresponds to the vertical position of the baton. This would mean more processing of the information by the singer, but is trivial to build electronically.
An Arduino would be perfect for the processing/driver.