I am developing a web app for stock analysis in which registered users will be able to write blog posts.
This is not the core feature but I believe it will help with the marketing in the long run.
Should the blog feature be part of the MVP thus available on launch day?
If it's not a core feature then it doesn't go into MVP. By definition its Minimum Viable Product.
It makes sense, how would you share (better say market) the site then? I am talking about both S.E.O. and not
For a start, I don't use the term SEO. In my eyes it is the sleaziest market sector on the internet.
Explain your product to your target users in the language of the problem domain with clear non-waffle content and let Google et. al. do what they do best.
Then make your product stand out so that all these stock analyst users will blog about how awesome it is on whatever platform they are currently blogging on (if they are not blogging on another platform already, then you are going to waste even more time figuring out how to convince them to blog on your platform).
There's no "trick" to organic marketing. Make a product that is better than the competition. Invite a few prominent figures in the industry to try it out for free - no strings attached - and see what they say.
If it's good, the news will spread and you're on the path to organic growth. If it's bad, you've got yourself some first hand customer feedback.
The only time you need a whopping fat marketing budget and plan of action is if you're product is just "meh" compared to the competition and you need to convince paying customers why your "meh" is better than the competitions "meh".