I applied for BC/BS on 8/24/ 2007 and accepted on 8/ 24/2007 effective date of 9/4/2007. I was never late on a premium. I had surgery on 12/23/2008 at Northwestern Memorial Hospital for kidney stones; they denied coverage on 12/26/2008 and was told I had a pre-existing condition.
I had a previous CT scan on 8/13/07 and informed in mid-September 2007 that I had kidney stones. This was several weeks after I accepted coverage on 8/24/2007. I wrote to BC/BS on 2/21/2009 to PF on 3/05/2009 who wrote they would review my case. On 3/12/2009 PF wrote after additional review nothing changed with their original decision.

It sounds as if you MAY have had a pre-existing condition you were unaware of, although the fact you had a diagnostic test just before you applied suggests you had symptoms you were aware of. If you did not report them and the test on your application you may be out of luck.