Well, first the bad news, Sig is no longer marketing that gun as the "Blackwater" so having a completely original Blackwater model would be kind of cool to have and not having the original stuff is going to hurt the value to someone who knows what's what.
As for what you gun is worth, that depends on multiple variables, how many existing mags do you have for it, how much use has it seen, has it been well maintained? Do you have the original box and papers etc.
A current 226 "Tactical Operations" (What used to be the BlackWater model) has an MSRP of $1300, that would put average retail around $1000-$1200, subtract $200 for a used gun, your looking at $800-$900, subtract original materials, pending on what's missing you're looking at $50 or more being taken off, that's if the gun's in great shape with no major holster wear or scratches. I estimate you've got a $650-$800 Sig given the current market, pending on the overall condition of the gun of course.