A special category for blind lower secondary students has been in the Bebras challenge since 2013. During its existence we have adapted 63 tasks for blind students. But in many cases their difficulty was not correctly determined. We have analysed the influence of various factors on the real difficulty of tasks. We have discovered a significant influence of affiliation to the thematic area. It is also important what cognitive operations the competitor must perform to solve the task. We also observed a lower success rate of younger blind students. It has not been confirmed that the length of the assignment would have a significant effect on the difficulty.