The bottom-most facets on the diamond (the main pavilion facets) draw their light from the highest angles. When a Hearts & Arrows diamond is photographed from directly overhead, these areas will go dark if the camera blocks, or obstructs, the light source. This is known as obstruction. The dark areas are, literally, a reflection of the viewer or camera lens in the crown of the diamond.
Will I see dark arrows in real life?
Not unless you work hard to do so. The only way obstruction can be seen as it appears in these photos is if you are able to hold the diamond perfectly still while blocking the light source with your head at the correct distance. When diamonds are unobstructed (practically all of the time) the patterns are not obvious.
In the first photo the camera is positioned so the arrows effect is visible in the center stone, but the side stones are offset, so the arrows are not visible. The other photos show different angles and the resultant contrast patterns. All of these diamonds have premium optical symmetry, but for the arrows effect to be visible the camera must be directly over the crown, blocking the light source at the correct distance so that all of the pavilion mains go dark simultaneously.
Diamonds with premium optical symmetry show symmetrical pavilion mains and have balanced performance qualities when cut with great light return. Many commercial diamonds do not enjoy such precision. Diamonds with different levels of cut precision have different obstruction patterns. Predictably, this influences the consistency and character of the performance.