I ended up using Reddit for blog comments:
http://www.oilshell.org/blog/2016/12/29.html
Reddit has:
1) a good commenting interface
2) many existing users with accounts
3) a low barrier to a signup for those who don't (you barely even need an e-mail address)
4) an API
The downside right now is that you have to leave the page to comment, but I don't think it's a big deal. It probably lowers engagement a little, but I find that the most commens happen on other aggregators like HN, no matter which service you use for comments.
I also use reddit's RSS feeds, since a few people asked for that.
Reddit has its share of immature users and a culture of snark, but my subreddit has managed to steer clear of that.