The black blades are described as "carbon tool steel", they don't really get blunt but they do snap.
My guess with the black blades is that they are made from steel with higher carbon content.
Higher carbon content hardens steel, but makes it brittle.
The harder steel however can be formed into a sharper edge, whereas the softer steel can not.
BUT the harder steel will not hold their edge as well, because that edge is more brittle.
For example, on the extreme side, we have GLASS. if made into a blade, it could be much much sharper than any steel blade.. but it'd break upon cut attempt.
CUTTING is more than sharpness, it's a balancing act of many material properties.