"Full bleed" is a printing term where the print image stretches to the very edge of the product (no empty margin). Your trading cards are full bleed, while a typical business letter would have 1" margins all around (no printing up to the edge of the paper). Cuts on the paper can never be guaranteed to line up exactly with the printed area, so printers typically require you to send an image that extends beyond the design space by one-eighth of an inch all around. This extended area is called "the bleed", and it allows a margin of error on the paper cutting -- if the cut is slightly outside of your designed area, the printed image will still be "full bleed". So if your trading card size is designed to be 2.5 inch by 3.5 inch, you provide the graphics for 2.75 inch by 3.75 inch -- an extra 0.125 inch on each side of the actual design. My guess is that there was something wrong with "the bleed" area in your design files -- maybe you didn't provide a wide enough bleed area or it was missing entirely, so the engineer substituted a black border.
"Full bleed" is a printing term where the print image stretches to the very edge of the product (no empty margin). Your trading cards are full bleed, while a typical business letter would have 1" margins all around (no printing up to the edge of the paper). Cuts on the paper can never be guaranteed to line up exactly with the printed area, so printers typically require you to send an image that extends beyond the design space by one-eighth of an inch all around. This extended area is called "the bleed", and it allows a margin of error on the paper cutting -- if the cut is slightly outside of your designed area, the printed image will still be "full bleed". So if your trading card size is designed to be 2.5 inch by 3.5 inch, you provide the graphics for 2.75 inch by 3.75 inch -- an extra 0.125 inch on each side of the actual design. My guess is that there was something wrong with "the bleed" area in your design files -- maybe you didn't provide a wide enough bleed area or it was missing entirely, so the engineer substituted a black border.