I was told the black square goes back to the days when very low power scopes were the norm, and it was the only positive point of aim given the optics in use at the time. I was cured of any interest in shooting the black square early in the BR shooting. I was shooting sighters using the black square as the aiming point, and somehow I lost a shot vertically over the border, and almost into the record portion of the target. After that, I have never really been comfortable aiming at the square.
There are a few things to consider about point of aim, and also where you want the group to form. There are multiple opinions on this, with some preferring to have their shots impact at point of aim, and others preferring to have the shots impact above or below point of aim. I have tried both, and I prefer to have the shots impact below my point of aim. I find that having the shots impact on my aiming point is too distracting.
Deciding on a point of aim can be influenced by the scope reticle. A dot, especially the 1/8 dot in a Leopold scope centres up pretty well in the centre ring (mothball) of the target, whereas a fine crosshair can make bracketing the ring easier. You also want a point of aim that lets you use the rings for hold off (IE the black square gives you very little to work with in this regard).
Lastly, you need to think about the habits you form while shooting the BR target – they can either help or hinder you. I like to use the mothball and hold on either the 9:00 or 3:00 position, depending on the wind (IE hold on the 9:00 position for a left-to-right wind which will let the group form around the centre of the mothball). I use this hold, unless there is enough push in the condition to move the bullet impact half or more of the target (more relevant at 200). In that case, I will hold on the 10 ring (outer most ring of the target) or even hold on the border if needed. This is what I mean about habits – there is not too many different holds to remember.
To give an example of how habits can trip you up, another shooter was at a nationals match a few years ago, and shooting in bad mirage. He decided to hold on the black square for better visibility, and ran the group pretty fast. On the 5th shot, old habits kicked in, and he held on the mothball. Instant 2.00 inch group!