The question here is; what is the idea of that blog?
If you want it to closely represent the opinion of your company, then you should host it and design it as one with the rest of your website, that way it always feels like it's the same.
If you want to use it as a communication/conversation mean, then Medium doesn't seem to provide that, so it's not an option.
If you want to write some content, but you don't care/mind that it is associated closely with your company, then Medium is a good option since it will attract readers who may not have the slightest idea who wrote the article since that information, although present, is not very prominent.
If you don't mind about the correlation between the blog/article design and you company site, then it may be an option, although remember that design is one of the elements to make something belong.
If you are interested on trendy design and don't have the time capabilities to make/install what you want, then Medium is a good option.
I can go on with a few more elements, but the general idea is, if you want the blog to be perceived as something that belongs and represents the company, then it should be on the company site/domain.
Also, you could present the content of an article or even the whole blog hosted on Medium inside your site, but I still recommend having it 100% on your site/domain/server.