Haar Training with object Blacked out in Negative Samples
I have had good success doing this with my training sets. I have modified the OpenCV_Annotation application to present a fixed rectangle that moves with the mouse, once the space bar is pressed the coordinates are written to the positives.txt file and a copy of the image is saved into a pre-defined negatives subdirectory after graying out the rectangle. 
I have seen no training problems or performance issues although I use other negatives as well as the blurred out images. The application uses a gray background to overwrite the identified area instead of just blacking it out. I am unsure if that would make a difference in the negative.