A Sharp Edge

An orange ball in a blue background
After blending, the edge is fuzzy. You can tinker around with sharpening your sticks or using the edge of a broken stick, but it's still going to be fuzzy. Note that the blue is darker. This is because the pigment has been pushed into many of the 'holes' of lighter paper that optically blended with the blue above to make a lighter-looking whole.
Pastel is water-soluble, so you could use water to 'melt' the pigment with a paint brush, but I use isopropyl (rubbing) alcohol, because it dries very fast and the paper has less time to buckle. Note that the blue now looks even darker, for the same reason as above; fewer or no light specks of paper-peekthrough.
After the alcohol has dried, you can still add more layers of pastel on top, but the dissolved and dried pigment has lost some of its bloom. It looks rather like poster paint.
.