In the Star Trek: Deep Space Nine episode “Way of the Warrior”, General Martok met with Benjamin Sisko and Kira Nerys aboard Deep Space Nine. Before they began the meeting, General Martok insisted on a blood test: All three characters cut their hand with Martok’s knife and dripped blood on the table. Since none of the blood reverted to changeling goo, it was assumed that all three were who they appeared to be.
This appears to be an effective way of screening changelings, and is later used to reveal one. However, we later find out that
Martok was a changeling, and had been the entire time - since before his first appearance until his death in episode Apocalypse Rising. The real Martok does not appear until the later episode Purgatory's Shadow.
Since his blood did not revert, how was this achieved?