From Parkerized to hot blue??

Yep, but the parkerizing has to come completely off first. Then send it out(expensive). A good bluing job is dependant on the polishing. It's all hand work and that's the part that costs.
For a trigger guard, I'd polish it myself (after removing the parkerizing by sand or bead blasting) using jeweller's rouge and a buffing wheel and cold blue it.
 
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