It's much like color contrast, and not a simple thing. For example, two squares of intense opposite hues might be precisely the same size, yet be perceived by us as differently sized, and with one advancing toward us, and the other, receding. That is a physiological as well as psychological phenomenon. But instrumentation would tell you something quite different.
Analogously, in black and white work, much of what we perceive as sharpness is due the nature of the adjacency itself, not only the differences in contrast, but also more subtle things harder to quantify, but very useful in compositional strategy nonetheless, because the intended endpoint is not a film densitometer, but human viewers having complex interactions of physical and psychological properties.
bluechromis - unsharp film masking CAN be used to enhance edge acutance, depending on how it is applied. But in color pre-press work, it's main function was color correction along with overall contrast control. Sometimes different masks controlled different aspects; sometimes a single mask could accomplish dual or even multiple purposes. With dye transfer printing, it could get complicated. Ciba was generally a one-mask problem if one knew how to effectively control more than one issue in as single mask, but it too could potentially get complicated, requiring multiple masks to achieve an ideal endpoint. Even some black and white printers use more than one mask relative to some special trick or another. But in terms of edge effect, the nature of the diffusion sheet in relation to the angle of incidence of the exposing light, allows a considerable range of fine-tuning edge options : how far the halo spreads, how harsh or gentle the transition, etc.