Because they are both used color is hard to compare. One of these may have been exposed to too much light or some other environmental exposure. Of course, it is always possible there were two or more printings?
Bale Israel catalogue states that the 20 value of this stamp is printed in both black and blue shades. It is hard to tell from your scan, especially with usee stamps, but shades are recorded.There are also phosphor varieties of this series which can also be found.