Hi Chelsea, you could set up a triggered smart campaign that is looking for the data value of your blog field to change to true. If you want, you can filter it down to specific forms/pages visited, if you have this occurring in other places and don't want them to receive the welcome email and go into the blog digest list every time the data value changes to true.
One other update you will probably want to make is that they can run through this smart campaign every time.
Let me know if you have any other questions about this!