Tim, pineapple juice is better for cleaning a rusty blade.
Most of the boldly laminated Moro blades shown by well-known sellers have been stained with ferric chloride; while impressive, it may be argued that this stain is not traditional.
This blade was etched with hot vinegar:
http://www.vikingsword.com/vb/showthread.php?t=6792
Vinegar is the main active component of coconut water - apparently the traditional cleaning and etching fluid.
Keep the etching time short: the blade should stay smooth and not corroded as in keris Jawa. You may have to do several rounds of polishing and etching to get a decent staining, especially if the blade has been polished with power tools by some former owner.
Regards,
Kai