Hey, if you haven't figure out this already - you have to open your store in order to allow public access to all parts of your Shopify site including the blog. That said, what you can do is you can hide all navigation access to your catalog / product collection + make sure all your products are unavailable so access to the commerce part of your store is unavailable.
1. Choose a paid plan and launch your website.
2. Rmb to turn of storefront password under Online Store > Preferences > Password Protection.
3. Remove "Catalogue" or other product link under Online Store > Navigation
4. When creating a product, make sure to uncheck Online Store under Product Availability
5. Now you can go post on your blog
When you are ready to sell, just go to step 3 and 4 above to insert navigation URL and check Product availability for Online Store.
Hope this helps