***** NOT A TEACHER *****
Good morning, kiweep.
(1) This is something that confuses native speakers, too.
(2) Most books accept a) as "correct."
(1) Blake is one.
(2) One of what?
(3) Of the students.
(4) What kind of students?
(a) The students who SING in the concert
(5) Blake is one (of the students + who sing in the concert).
(6) Yes, I believe that your second a) with "the only" does, indeed, mean what your friends have told you.
(a) Just mentally erase "of the students" and you get:
Blake is the only one (who SINGS in the concert). = adjective clause that modifies "one."