In a recent paper "The black thread and other repairs?", linked recently on shroudcom, Pam Moon considers the matter of a dark thread visible on the photo.
I just would like to remind similar things have been reported for a long time. They may clarify the issue. First in a paper Joseph G. Marino, Edwin J.Prior, Chronological History of the Evidence for the Anomalous Nature of the C-14 Sample Area of the Shroud of Turin have a following note:
Data Category: Evidence of anomalous nature of C-14 corner and C-14 aspects
Evidence: Even though Riggi had given assurances that the excised C-14 samples given to the
labs were free of foreign threads, The University of Arizona, one of the laboratories that
performed the Shroud C-14 dating, documented both red silk and blue satin in its sample.
Source: Petrosillo, Orazio and Marinelli, Emanuela. The Enigma of the Shroud: A Challenge
to Science. San Gwann, Malta: Publishers Enterprises Group, 1996, pg. 86.
It has been also mentioned in Marinelli’s paper from the Valencia conference:
In Tucson a thread of red silk and blue fibers were found on the sample238 which points to the Sox as original source of information 238 D. SOX, How an age of mystery ended, in The Times, October 15, 1988, p. 36.
According to the scheme from Ian Wilson’s 1998 The Blood and the Shroud (scan here, published by Stephen E. Jones on his blog). According to the scheme there was blue surrounding frame sewn onto Shroud by Princess Clotilde, 1868 nearby, as well as her stitching on the edge of radiocarbon sample. I presume she is the main suspect for leaving such dark threads in the radiocarbon sample. I don’t know whether that means she made some manipulations in the radiocarbon sample. It may indicate towards this direction, as well as those traces might have been accidentally left there during her work on the Shroud.