In Mishna 2 of Perek 1 of Terumah, it is stated that a deaf person who can speak but not hear should not separate Terumah, but if he does the Terumah is valid. The commentators state that this is because he is unable to make a beracha that he can hear.
But this makes no sense to me. After all, a deaf person (who can speak) may eat, and eating also requires a blessing. What is different about Terumah that makes it forbidden for a deaf person to separate it?