You have a typo in your HQL-query use 'COUNT(a.b)' instead of 'COUNT(a.c)'.
What SQL-query would you expect from such a HQL query?
I don't know of a general solution to this problem but I guess it will involve coalesce and concatenation.
IMO a naive solution would be to create the following SQL query:

select
    count(a0_.b_bid || a0_.b_bname) as col_0_0_
from
    A a0_