Just as a mental exercise, it seems going to a local coffee shop would make me easily spottable on CCTV, especially if they can correlate visits and posting times...
Depends if Tor has done its job and concealed the IP address you're posting from.
Right now it's 10:30am in Iceland and 10:30pm in New Zealand so the fact you're posting at this time of day, and drinking coffee at this time of day, probably doesn't uniquely identify you.
Indeed, I'd say always post at the same time of day, rather than posting at random times when coffee shops are open, as the former does a better job of concealing your timezone.
(Obviously, if you're some Dread Pirate Roberts type and Tor doesn't do its job, it's game over.)
You could schedule posts so times will not correlate. Tumblr has a queue which you can use to post at regular intervals throughout the day.
I guess they could try using a blogging service hosted in a "non-cooperative" country. E.g. Russia, unless you want to blog about Putin.
We could keep playing this game forever. Really you'd want a self-hosted site running from an anonymous VPS paid using a cryptocurrency like Monero (these exist) and your domain would use a similar service (say Njalla) or be TOR-exclusive. Alternatively you could go a more classic route and host with SDF (à la http://voidnull.sdf.org/).
Or, if you're really paranoid, you'd give up on the whole thing. Maybe you'd still write but you'd keep it to yourself. As I see it, any interaction is a risk of exposure even if very small.