The effects of BFR on tendons has not been studied in humans with either normal tendons or injured tendons or tendinopathies.  There is a study in horses that showed muscle hypertrophy and no change in tendon thickness.  The problem is that an improvement in tendon tensile strength is not related to tendon size or thickness, so it has proved difficult to study the issue.
However, literally millions of BFR sessions are performed annually around the world and there are no reports of tendon ruptures.
Further, there is evidence that muscle is improved and there is evidence that bone density is improved by BFR, so it is likely that the structure in between those two tissues is likewise improved.
Finally, patients/clients with tendinopathies benefit from B Strong BFR sessions by improving their overall fitness and reducing pain in their tendons.