The problem is that not all ebony looks the same. I've seen some ebony fingerboards on some instruments in pure black, and some with gray streaks. I've seen some with very fine grain and some with quite course grain. When dyed black, some of the blonde hardwoods look just like some forms of ebony, which is exactly why people dye them black. I've even seen rosewood dyed black to look like ebony. Since often even ebony is dyed for a more pure black appearance, a person would have to work with ebony all the time in order to accurately recognize the difference without cutting into it, like for example, if a person was a double bass luthier.
Also, with a double bass, the fingerboard is one of the items that gets replaced from time to time. Even ebony fingerboards wear, especially with certain kinds of strings -- ruts emerge with playing, and the fingerboard is dressed a number of times for ruts or to accommodate different kinds of strings, and it becomes thin enough that it is time to replace it.
If it were me, and if I was looking at a specific double bass, having appropriately done my research about all of the other more important points, possibly including talking with the instrument's builder or taking it to a luthier to examine its condition, and if I like how it sounds, then at least in my opinion the type of fingerboard wood should not necessarily be the deciding factor for whether or not to acquire the instrument.