Wrong coloured lead used on replacement window

Back when I did some stained glass, there were two methods of doing the fitting.

• The "old" way, with lead came. This is typically an H-shaped long lead channel that the lead fits into on both sides of the channel, then the pieces of lead are soldered together at their joints. Then would be finished with a putty of sorts, worked into the joints to seal things up against weather and moisture infiltration. This method typically is not then finished to give the lead a different color, but rather typically is left the dull dark gray of the lead. This is still used nowadays, but not as common as the more modern method outlined next.
• The "more modern" way, is with copper foil. This method was popularized by Louis Comfort Tiffany for making stained glass lampshades. It involves wrapping the edges of the pieces of stained glass with narrow copper foil strips (sometimes adhesive-backed, sometimes not, depending on the era). The pieces are then fit tightly together, and all of the joints are soldered together all along the joints, covering all of the foil with solder. At this point, the solder is a somewhat dull silvery shiny color. The solder lines can then, optionally, be chemically treated to get to a shiny silver, copper, or blackened look, depending on desired outcome.

Without seeing a picture to be sure it's hard to know 100% what you're seeing and describing. But based on your description, I would be guessing that your repair person used the copper foil method, and then chemically treated the solder to give a black color. Whether it's possible to re-treat the solder lines to get the bright shiny silver color you're wanting, I have no idea. Your stained glass provider/installer would better be able to answer that question.