She studied Tel'aran'rhiod in detail for at least fifty years, as well as many ter'angreal related to Tel'aran'rhiod and Dreaming, in particular the sleepweaver that Egwene uses to enter Tel'aran'rhiod before her training with the Wise One dreamwalkers and later on by Nynaeve al'Meara, Elayne Trakand, Siuan Sanche, Leane Sharif, and eventually the rest of the Aes Sedai in Salidar. Verin Mathwin found the ring and a collection of her notes only by chance; all records of the ter’angreal had been removed from the official records. Verin thinks that Corianin was a very secretive woman. She kept most of her knowledge to herself, leaving most of what she had learned only in her notes.
This brings into question how much information Corianin withheld from the White Tower in her research. Given that Verin withheld the notes at the same time she was assisting Egwene in her search for members of the Black Ajah, Verin could have been caught between her Black Ajah oaths and the Amyrlin's command to help Egwene (violating which would have also meant violating the oaths). In that case, it is entirely possible Corianin was herself Black Ajah and that her notes proved it, or that somehow the notes, or how she came to possess them, would show Verin to be a Black sister.