Why do new helicopters have rotor blades with swept tip?
Helicopter rotor design is, among many other things, a balancing act of retreating blade stall and transonic effects on the advancing blade.
Helicopter blade tip devices usually serve to reduce transonic effects on the advancing blade. The Ka-50's blade tip is rather simple (only swept) as the coaxial design is affected less from these transonic effects, as they happen on both sides. For the same reason, retreating blade stall is also a lesser problem, so the designer can get away with a slower blade tip speed.
Helicopters with a single main rotor don't have this luxury, so many of them have complex blade tip devices with sweep, larger chord and sometimes different airfoils. I believe a good example is the Westland (now merged, AgustaWestland) Lynx.

Blade leading edge is top of picture. Sweep, increased chord and droop on end are evident.
For my Bachelors thesis, I was looking for blade tips with the highest drag divergence Mach number, the Lynx blade tip's was M 0,91. I believe this is an important reason why the Lynx is the fastest conventional helicopter.