Is black thread weaker than other colors?
Only if it has been over-dyed because the first color was mis-dyed. Some companies put all the mis-dyed colors in a reject bin and over-dye them black. The excess amount of dye in these over-dyed threads can cause the fibers to weaken. (Note: Superior Threads are never over-dyed.)
Reprinted with permission from Bob Purcell, Superior Threads