Yes, there are a few blogs written by professional speechwriters. I’ll list eight of them by their author’s names in alphabetical order, along with a sample post.
[Dr.] Sam Cooper just uses his name . He has a post on February 4, 2019 titled Daniel Kahneman’s Thinking, Fast and Slow: a treasure trove of tips for speechwriters and speakers.
Charles Fleming has a blog titled Expression/Impression . He has a post on October 11, 2021 titled Keynote speeches: tips from politicians.