Nested type in Elasticsearch: "object mapping can't be changed from nested to non-nested" when indexing a document
It seems you're really creating a document of type some_type and comments will default to a normal object (i.e. not nested), which is not allowed since you already have a nested object called comments in the blogpost mapping type in the same index.
Try this instead and it should work:
PUT /my_index/blogpost/1{ "title": "some_title", "comments": { "name": "some_name", "comment": "some_comment" }}