I have final portfolio review tomorrow at 1730. I went out looking for spotting fluid, but every art and photo store in town is out. All I have is olive Spot-tone.
I think I remember ed weston spotting with india ink and gum arabic. Not sure if those are readily available. Here is a link from a google search: http://silvergrain.org/w/Spotting
Peerless aniline dye - the book of blotters loaded with the dye. Mix up little puddles of water with a tiny chip of the blotter(s) in a dish. You will have to blend the dyes for neutral - warm - cool blacks. Don't try to use it straight, it's way too strong.
I ended up coming up with a creative solution and it worked for the most part. I used a simple Rose Art black colored pencil that I rubbed the point off of. I did the marking before I put the prints in the dry mount press. When they came out, the wax had melted and worked into the emulsion. It may not be archival, but it got me through final critique.
Try good stick sumi ink; make a dark ink and then dilute it to the correct value with water. Sticks are avaliable in cold and warm blacks and dried inks on a stone or tile can be rewet and used.