Which compound or compounds are responsible for color change in blue butterfly pea flower tea?
OK, that was simple. This

https://www.google.de/search?q=Clitoria+Ternatea+blue+color

gave this

https://www.ncbi.nlm.nih.gov/pubmed/14568080 

which lead me to this

https://en.wikipedia.org/wiki/Anthocyanidin

and I guess that's the culprit. I judge from that abstract linked above that the actual plant can contain a wide mixture of those anthocyanidines.