Suppose we have a blog with valid https certificate and we have some comments on our posts with some outgoing links which don't have a valid certificate, Google will not rank the page as secure? does this mean that comments left by http sites on https sites will negate the point of having https, until all commenting blogs use https?
No, I don't think so, Blog comment links are mostly made nofollow by the blog owners. So, its won't imapct your site SEO. Just make sure that all the images and urls of your post, pages etc opens with HTTPS.