Don't let the term 'blacklining' fool you ... it doesn't have to be black paint ... in my opinion the term 'darklining' is much more fitting. Black is an option, but you could also use a dark purple, brown, blue, ...
This technique adds definition by visually separating different colors/areas on the model making it much easier to read.
As to how to do it ... there are many ways. I wouldn't use ink as inks have a tendency to be very shiny. Just use 'regular' paint, thinned with water. Just make sure to unload you brush on a tissue or similar as you don't want to flood the miniature with paint. You want the paint to flow easily and controllable off your brush.
You can also use darklining at different stages during your painting process. Some people tend do do an overall black-/darkwash after priming with white, starting with the darklining. For others it is the last step before finishing a miniature. For me personally it is part of the shading process. I often use the shading color I have on the palette for darklining, coming back at different stages de- and refining what I have already done. Also when you shade with glazes/washes you get an automatic darklining effect just by pushing the pigments.
Hope that helps
