Have any countries used blockchain voting in national or local elections?
Sort of. There have been a few trials, the most prominent of which was the usage by the state of West Virginia in the 2018 midterm elections, which saw 144 overseas voters vote using a phone app which utilized blockchain technology.
However, in 2019, the trial was denigrated in an article by Slate, which pointed out that officials had yet to share the information required to evaluate the success of the trial. In addition, it points out that none of the promised security auditors were ever named, nor the scope or length of the tests they supposedly conducted. The FBI also launched an investigation into hacking attempts which occurred during the trial.
In Switzerland, also in 2018, the city of Zug conducted a trial of blockchain based voting which was hailed as a success by administrators, but the vote was just a test using invented questions, and the results were non-binding.
Sierra Leone was widely reported to have conducted their 2018 presidential election using blockchain voting, however this was later denied by the country, and many outlets issued retractions and apologies.
In conclusion, while trials of blockchain voting technology have been performed in some elections, only the trial in West Virginia has actually used blockchain technology to collect votes in a legally binding election, the trial only applied to a tiny percentage of the electorate, and the security considerations behind the trial are murky at best.