Nirvana Black Oil appears to be the Warwick name for that color. The finish looks like a black wood dye with an oil finish. I do not know what type of oil finish Warwick uses though - is it natural oil or a thin varnish? My bubinga Corvette does indeed have an oil finish (though I assume they use a curing accelerant and additives as it maintains its sheen very well).
Assuming that you have a true oil finish and not a varnish, any solvent that will melt the cured oil will also likely drive it deeper into the wood. Also, If the black color is indeed a wood dye, solvents and strippers will not help you strip the black. Natural oil and dye, however, can usually be removed by sanding without too much trouble, unless they used a process to really drive the finish deep in the wood. The finish might not be very deep in the wood, and I bet light sanding with 320 grit would take it off the bass (no pressing too hard to avoid deep cuts in the wood).
Edit:
Removing the strings short term will not mess up the neck. It should be fine during the time that you refinish the bass. (I might loosen the truss rod a bit, but unless there is an inherent problem with the neck, a couple of weeks without the strings should not be an issue.)
As far as finishing goes, if you want the Warwick wood look, you have several options for refinishing. If you want to add color to the wood while retaining all the grain, an aniline wood dye, either water or alcohol based, would work well. You can then put most any type of wiping finish you like on the bass.The bass can be coated with either a varnish or an oil. Keep in mind that most products labeled as "oil finish" are actually a thin wiping varnish.
A wiping varnish that is satin or low luster will give you an oiled look to the finish. These are available as either oil or water based. These are easy finishes to apply. Most of these on the market are urethane based. There are still a couple of phenolic based varnishes available (my preference), but they are expensive and might be a bit more challenging to use if this is your first project.
You can also choose to go with a natural oil finish. Any drying oil will work. The two most common choices are tung oil and linseed oil. Many stores carry a product that is called, boiled linseed oil (blo). This stuff has an advantage in drying time over natural oils. However, blo will yellow as it ages, so this should be a consideration if choosing to use the stuff. Natural linseed oil will also yellow slightly over time. Tung oil will yellow the least, but it will show a slight darkening over time.
The big disadvantage with natural oils is their curing time. Once you finish applying them, they should be given at least a month to cure sufficiently for handling. I prefer using pure tung oil. It is easy, but it requires patience.
For dense woods, I start with a mix of 1:1 of tung oil and mineral spirits. I apply a coat, let it sink in for 15 minutes, wipe down the wood, and wait 24 hours. I then apply the thinned mix once again and wait 24 hours. Next I apply unthinned oil, let sit for 15 minutes, wipe down, and wait 24 hours. I keep doing this until no more oil is taken in by the wood. This is usually 4 to 5 applications. After about one week, I'll hit the finish lightly with extra fine steel wool. I do this every few days, and within 4 to 6, it's a super pretty oil finish.
If yer going to the trouble of a REFIN, why not "go the whole hog, including the postage" and do one of these paintjobs?
