I'm not sure how I missed this and I'm not sure how competent I am, but here are a few thoughts. I picked up the harp about 5 years ago and found it fairly easy, especially after having played guitar and having studied a little bit of music theory. The original online course that I did was by Ben Hewitt. I think it is one of the better ones out there in that much like Griff, he approaches it from a music theory standpoint and it has a very comprehensive scope that covers everything from square one, up to some advanced techniques. It is not specifically a blues course, but a large number of the examples are blues-based. You can watch a couple of the lessons on the preview page at:
https://www.udemy.com/course/benhewlettharmonicatuitionultimate-harmonica-course/
Another good source is Adam Gussow's website at
https://www.modernbluesharmonica.com/home.html. Adam is a literature professor in Mississippi, who has really studied delta blues, and is also an accomplished harp player and teacher. He does not have a comprehensive course like Ben Hewlitt's, but has really good resources for learning individual songs or techniques, as well as very good song transcriptions to harp tablature. There is also a Facebook group for Gussow students, which also has a number of pro players like Charlie Musselwhite and Rick Estrin who sometimes post.
The best thing about playing harp is that I can carry around a practice set in my car for impromptu practice in traffic jams, while waiting in parking lots, etc. Like guitar, there is a huge array of gear available and the predictable debates about harmonicas, amps, mic's and other gear. My best suggestion is to start with a reasonably good quality harp, which is typically in the $25 to $60 price range. One of the issues with new harp players is that there is a tendency to be a bit rough and blow a lot harder than necessary, which can result in broken reeds. I started out using a Seydel Session Steel ($50-$60) to learn on, since it is German-made and has stainless steel reeds, which can stand up to a lot of abuse, but sounds great. For comparison, Seydel Blues Session with bronze-phosphor reeds is in the $30 range). Regardless of choice, a good harp in the key of C is the first one to get. That will allow you to play in C in first position, G in 2nd position and D in 3rd position. Add more harp keys as needed.