Deny user to see any database through DB_NAME()
DB_NAME does not work as advertised before SQL Server 2016 (where the behaviour of DB_ID is also changed). For details, see:

Information disclosure with the db_name and db_id function (Connect bug report)

There is a similar situation with other metadata functions, including:


suser_name
suser_sname
suser_sid
user_id
database_principal_id
is_rolemember
is_srvrolemember


These issues remain unresolved (either by design, or by being 'fixed') as of the time this answer was written, as far as I know.