Using et al. for a last / senior author rather than for a first author

I think in an informal medium like a blog post, you could convey that author D is the main character in the “story” you are trying to tell while still maintaining proper citation standards by saying something like, “Author D and his/her colleagues (Author A, et al., 2018) have shown that...”

Your feelings and the journal citation rules might not align the way you want them to be. Therefore, I suggest you stick with the accepted norms and go with author a et al., even if it is an informal conversation or blog.

Using any author other than the first author in citation would not only do injustice to that author, but would also be a disservice to the readers. Readers would probably be confused when trying to find the bibliography of that citation in the reference list.

The only situation when this is common is if you're referring to multiple papers at once. However, even then you'd probably want to use formal citations in parentheses (like mentioned by LarrySnyder):

LAST AUTHOR et al. have done a lot of work showing phenomenon X (AUTHOR A et al., year; AUTHOR B et al., year).