Jason,
I've seen wrenches soaked in oil, then layed out in the sun that took on a nice, dark blue/black color. I have a friend that does that to his. This is after a vinegar soak and brushing. You might try that on this one.
I've also heard of folks cleaning, then artificially re-rusting tools with a spray of low concentration salt water. Then, leave alone until a thin layer of rust appears on tool. Then a light, fine brushing after an oil soaking and a pretty nice patina results. You might wanna try that also.
Just suggestions for ya, your mileage may vary.
-aaron