Database and query optimizacion
I need to verify that the number that I will insert into table tbl_database is not in tables tbl_blacklist and tbl_cmdatabase


Perhaps you can do this with something like 

SELECT count(*)
FROM (
  SELECT number FROM tbl_blacklist
  UNION
  SELECT number FROM tbl_cmdatabase ) AS Q
where Q.number = '$number'


When the count returned is 0, it means that $number is not in the tables. It's possible this is not the best query for this job, but I think it will work OK.

You can play around with it here:

http://sqlfiddle.com/#!2/7d697/23