They're supposed to be but sometimes they can be less than perfect especially with dark hair. Worst case scenario is thin densities with black or dark brown hair with the factory bleaching not quite A1. You can bleach them yourself using Debbie's kit or another proprietary off the shelf bleaching mixture some of which are more successful than others. Or you can send it back to TL to rebleach them, or to Debbie.

Be warned though that bleaching can weaken the knots which, unless they are tied into the base very well can loosen leading to rapid shedding. One reason why some of us order blonde hair and dye it ourselves. Of course the problem only exists on lace bases, nit skin or Ilace, and then only if the factory doesn't do its job properly.