I have spent several days off and on looking for a good introduction to blockchains and cryptocurrencies for someone with mathematical background but no specific computer science or cryptography background. I've had no real luck. All of the books I find are either targeting investors, who don't care about the mechanics and just want to make money, or experts, whose language I am unfamiliar with.
For example, I don't really know what a hash function is, but I am totally comfortable with number theory, combinatorics, probability, etc.
What is a good introduction to blockchain and cryptocurrencies for someone with a reasonably strong math background, but no domain specific knowledge?
I'd change your search to be on cryptography, rather than blockchain, as the hash function is where all the interesting maths is. There seem to be quite a few online lectures searching for "mathematics of cryptography". Just browsing books, Applied Cryptography by Bruce Schneier gets high reviews, and is apparently quite mathematical (including reviews telling you not to be afraid of the maths).
The rest of blockchain designs are the practical issues with having a distributed database. That could potentially be a very long rabbit hole. Outside of the obvious suggestions of books and tutorials on YouTube, the Morning Paper blog covered quite a few papers in this area, as one of his main interests. But this is aimed at computer scientists.