the customer thinks the seam tape bled.
as far as this goes, although it seems logical at first, you wouldn't expect to see an even strip all the way across mainly because the layer of adhesive wouldn't allow for it
the heat damage from installation seems more likely. The iron/tape itself or the seam weight/toolbox tray put on top trapping it
edit
From Roberts literature:
PREVENTING SEAM DISCOLORATION
• The most common cause of discoloration is a seaming iron that is too hot. Roberts 482G Air Cooled Heat Bond Iron is the optimum solution. It features a double heat shield that reduces the temperature of the top shield, which comes in contact with the carpet backing. Iron setting should be at 3 or 4. If using non cool-top irons, use a plastic heat shield to shield polypropylene carpet backings from excessive heat.
• Always seam with the direction of the nap. Seaming against the grain can reverse the pile in the seam area. This area will reflect light differently than the rest of the carpet and appear discoloured.
• Metal trays and other weights constructed of plastic, glass, marble, etc. prevent residual heat and moisture of freshly made seams from escaping. Trapped heat can cause carpet tufts to lose their pre-set twist and trapped moisture can cause tuffs to swell, resulting in discoloration and pile reversal. Use a seam weight that is designed to absorb and release built up heat and moisture