Thank you for your help. Following your suggestion and installing the plugin disable-emojis solved my problem.
The site I’m testing now is on a local XAMPP stack, but the public blog at http://pettypool.com/blog/2015/08/joshua-craig-poole/
shows the problem. I’ve tried changing themes, and all behave this way, so I think that it is some kind of WordPress issue.
On the local XAMPP version, I can see the “correct” arrow, obeying my CSS, flash up, then get overwritten by some kind of post processing by WordPress which makes the “ugly blue” arrow. I’ve tried to trace it using the web browser’s “show source”, but it exceeds my limited knowledge to fully understand what is happening.
