asbestos

Yup, it sounds like it's the water, water turns oak black for some reason. The stains can be removed with chlorine bleach, oxalic(sp?) bleach, or that A B two part bleach. I have not tried oxy clean or the like but that may well work. Of course don't mix these products and read the labels. The issue is you will take the black out but also take out some or all of the color of the wood, so your matching concerns are valid. By messing around with various shades of stains and such (try using just the liquid at the top and/or just some pigment at the bottom of the can) you may well be able to get a good match. just keep tring and remember if the color is not right you can wipe the stain off over and over.
one note here, do not leave the rags with stain on them inside or anywhere where them catching on fire is a problem, this goes for many oil based finishes. again read the label.

milehigh_woodcrafter

pretty sure water doesn't turn oak black, it's water with contaminants that turn oak black, trust me i've dealt with it plenty.

as far as your job, you might be able to just sand them smooth with 150 roughing the existing finish. depending on the existing finish carrier (solvent) it may or may not work with your new stain and finish combo (depending on the carrier). It could be a whole nightmare, or rainbows and gumdrops. Somtimes life's funny like that.