You will find some informations about the curd test and the detected copper at the link http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3644753/

I dint get the exact answer but some hint ofcourse.

Thank You :)

Tamra bhasma prepared from "Tutiya" i.e Copper sulphate (Ref:  Book :Ayurved Sar-sangrah, By  Baidyanath Prakashan).Residue of CuSO4 is removed by " Shodhan Vidhi". The Aq  solution of Tutiya is blue and if the bhusma is not purified after shodhan vidhi and if there is still small residue of Tutiya in Tamra Bhasma then it turn curd blue.   Tutiya is considered toxic in Ayurveda.

When copper is oxidized by a combination of air, humid and finely divided acid and salts in the air, then a so-called patina of copper is formed. This is a basic copper (II) carbonate/sulfate mix, which has a cyan color.

simillarly when there is incomplete bhasmikaran then it means it have basic free copper in it that react with lectic acid of curd and produce cyan color. It is also an indication that when youprepare copper bhasma, the process increases heat ressistancy and make it resistant to chemical reactions. if bhasma is completly formed then it must not be reactive, so using curd in case of tamra is also a test of unreactivity of copper bhasma. 

if copper bhasma gived blue color, it means it is not properly formed and basic copper is in free form.

Curd test is one of the classical test mentioned to check the quality of bhasma specially copper containing. Most of the copper bhasma and other minerals having copper bhasma are in the form of CuO and CuS in final product. It is a test to check free copper metal, because when free copper reacts with lactic acid (one of the component of curd) copper salts are formed which gives blue colour and it is toxic in nature and produces nausea, vomitting beacuse of free copper ions, then again we have to go for further incineration process to pass curd test and get genuine quality of copper compound bhasma.