I must confess I did not recognize that icon from anywhere. Seems like a much simpler root cause is that many others also did not recognize this as a chat icon, particularly those outside of webdev or design circles.
I was coming here to say something similar. There is plenty of data out there that labels are often preferable to icons because people understand words, but don't necessarily understand your icon.
So it may be that there isn't blindness to the icon, just less confusion with the text. You could further test this perhaps by reinstating the icon and labeling it. That might convert even better.
