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Interesting indeed, Susan.
What "make" is the paint ? What does the label state as to pigment contents and vehicle ?
If the paint is truly "lamp black" (i.e., a mixture of soot gleaned from the burning of lamp oil, whether a non-drying animal or vegetable oil such as olive oil or tallow, or a petroleum fuel such as kerosene) it's the nature of carbon blacks to absorb one helluva lot of oil. If the vehicle is safflower oil (nearly universal in "student grade" colors) the combination could result in a mixture that truly wouldn't dry until the cows come home.
Here's a newsflash: Liquin is a painting medium, not a siccative (drying agent). It is inadvisable to use mediums strictly as driers. Siccatives such as cobalt or manganese linoleates, or lead naphthenate may hasten the drying of paint films, but must be used sparingly and knowledgeably to avoid paint failures. Incidentally, Liquin is compounded from alkyds (synthetic resins) and petroleum distillates and its use may result in delamination if you paint in a layered technique. (e.g., underpainting, over-painting, glazing)
Mars black or ivory black are preferable choices for most palettes. Synthetic iron compounds are the pigments used for those colors nowadays (no elephants or Martians have been burned). Their chief differences being that "ivory black" should be transparent, with a decidedly cool caste, while mars black is opaque, and may run a gamut of warm to cool undertones, depending on the source. As blacks go, both are relatively much leaner than paints made with true carbon black pigments, as the oil absorptive indices of iron-based pigments is much lower than that of pigments which are essentially, different kinds of soot.
As for sound craft in "building" a painting, it is inadvisable to paint lighter values over very dark values, (especially black from the tube) and for consistent, good results, painters should have at least a nodding acquaintance with the principle of painting "fat over lean", to apply to the process of painting a picture.
Neutral mixes of dark colors are more often than not, a better choice than using blacks from the tube. Burnt umber+prussian blue yields a "black" very nearly dark enough to be indistinguishable from black straight from the tube. To boot, this mixture is lean enough to use in underpainting, and dries very rapidly !
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