The two recommended free WordPress themes for starting a blog are Astra and OceanWP. They have all the features that you need to run a search engine-friendly blog. From SEO, load speed to mobile responsive.
OceanWP has over 700,000 active installations while Astra has over 1 million installations. Both themes have a free and premium version. They are also regularly updated.
The free version of Astra and OceanWP is available for download from WordPress.org.