A "stone washed" finish is very gently abraided. This gives the blade a sort of dull gray looking finish. But, this finish is very shallow and still easily scratched. These finishes are not easily repaired if you do get a scratch in 'em.
Bead blasting takes this even further. The blade is heavily abraided. It's done basically by sand-blasting the blade. The result is a very dull grey finish that is remarkably durable. And, if you do manage to scratch it, it can be fixed nicely if only you have the equipment (you'd probably have to send it in). But, unfortunately, many people, myself included, believe that bead blasting leaves the blade very succeptible to rust.
A black blade is one that has some sort of coating applied to it that colors the blade black. This could be anything from some cheap spray paint to a very sophisticated material that renders the blade virtually rust-proof (expect at the very edge where the coating ends). The problems with these coating is that they usually scratch easily and wear off.
Your BM42 has a sort of "brushed" finish. BM doesn't actually achieve it this way, but the classic way to create this finish is with a rotating metal brush. The result is very subtle little lines on the blade. These little lines leave the blade still bright silver in appearance but note quite so shiny and they tend to hide minor scratches. Minor scratches can be removed with sand paper or a brillo pad. These subtle lines to slightly increase rust susceptibility, but no where near as badly as bead-blasting.
Personally, I will not go bead-blasted because of rust. Depending on the coating, I may go that way, but probably not. I've graded to many balisongs with coated blades and had to seriously chop the value because of scratches in the coating. As soon as you coat the blade, that adds another category to my evaluation process and another opportunity to see the condition and the value of the knife fall. I'll probably end up settling for the sand washed.
Is Tachyon worth US$199 if you've already got a BM42? Well, you have to make that call yourself. If you like your 42 well enough, then maybe not. Tachyon is not necessarily better, but rather different. If there's some aspect of your 42 that you don't like, then maybe Tachyon will be better for you.
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Chuck
Balisongs -- because it don't mean a thing if it ain't got that swing!
http://www.balisongcollector.com