'Arm out the window' has got it right as far as the aerodynamics go. It also true that with negative blade twist, in forward flight, with cyclic pushed forward, the overall angle of attack can be negative.
If the blade pitch itself was negative, with respect to the horizontal, then just considering it as an aerofoil, with only a horizontal airflow approaching it, it would produce lift downwards. If you then add the upflow from the descent, as 'Arm out the window' said, the resultant airflow to the rotor approaches from an angle below it. The combined result of the relative flow from below and the negative blade angle is a negative angle of attack which produces a negative lift.
This is also true of forward flight, if the angle of attack is negative, so is the lift. With certain twist values and cyclic pitch, outboard sections of the blade will produce negative lift.
This is putting it all simply as the blade angle also affects the rotor speed which affects the angle of attack which affects the rate of descent which affects which affects the angle of attack.....helicopters, dont you just love 'em.
So, nope, you dont want a negative blade angle (set with the collective). As has been said, autorotations are fast enough with some postive lift, with negative...well, you'd only do it once.