NYDigger is correct. These were used from c. 1910 to 1921 by Hemingray until it was replaced by the CD 154 and even more common Hemingray 42.
I got into collecting insulators as a bottle collector too. My father has been a lineman for over 15 years. He took a few blue ones out of service that had been repurposed for power lines when these were usually used for telephone. Those ones are what got me into it.
Yup, Hemingray No. 40. Western Union adopted this as the standard in 1911, replacing the beehive (CD 145). They can be found in all areas and are very common, but it's a good start!