If i have no prior history of causes of blocked fallopian tubes(chlamydia, endometriosis, pelvic surgeries, etc), what could be the cause of blockage?

Cant say: There is not a way to know this. You could have been born this way. Most are caused by cases of chlamydia years ago that was never diagnosed or treated. There is not a way to be certain of the cause.

Reasons for blockage: Fallopian tubes may be blocked for a variety of reasons: 1) Infections, which are often undetected if no symptoms. 2) Congenital problem with proper development at birth 3) Temporary blockage via a tubal spasm during X-ray evaluation. Most of the causes have to do with scar tissue accumulation inside the tubes which blocks their opening, and hinders proper function, (e.g. Endometriosis, surgerg)