I have said it once and I’ll say it again: the color of the Noctua fans are, well, interesting to say the least, and I will deal with the color for the blessed silence.
The fan its self is a 7 blade design with an interesting tip design. It’s this tip that changed from the S12 to the S12B. In the design of their fans, Noctua says that the three aerodynamic parameters of airflow, pressure and noise play a major role. The proper mix of the three for the given application resulted in the S12 and P12 series.
The S12 series has a higher airflow/noise ratio with a low static pressure making it well suited for case fans. The faster air can be removed from the case the better. The P12 series has a higher static pressure/noise ratio making it well suited for CPU coolers with close fins. The higher pressure is needed to get the air through the fins.
The original S12 fan had a straight blade design. This design made for a fan that had an excellent airflow/noise ratio. To improve on this Noctua changed the length of the leading edge of the blades and the tip configuration. The added length to the blades increased the airflow and the static pressure as well as noise. To combat the noise they took the bevel tip design from their NF-B9 92mm and refined it, thus increasing the tip clearance, which in turn reduced noise.