Been training since I was 5 (I'm 29 now) and I'm from an EXTREMELY tough area (which I finally leave in a few months). Most people that hold a knife in a fight also use too much weight. A 'Jab' is the best use of a blade.
Holding a knife 'icepick' sty;e is great, but when I do it it is with the sharp edge facing me. This is great for slicing the other persons wrist/fingers without being sliced or stabbed. If your character implements this style, along with an obsidian blade beneath (stabbing style), you could have a nice and practical weapon. It might even work better if you have an Obsidian arc to cover the knuckles. You could literally punch slices into people.
(If you wish, email me and I'll draw some sketches for you.
noochint@gmail.com)
No need to make it fold. Give it a sheath you can punch in to.
Also, if the under-blade is obsidian and used for the killing blow, it could be replaceable, held in by a removable pin.
Don't think this is impractical for a second. Others may doubt it because it's too cool sounding, but the truth is there are already weapons like this, GO FOR IT!
PS: I wrote book where the 6 main characters (good and bad) used different and unique weapons. One of them even used electrodes on his fingertips (powered by a charge pack on his back). He fought multiple people by tapping different muscles, contorting his enemy to move as he pleased. A very interesting bad guy.