Blocking is great for keeping joists in line and needed on large spans, but relying on them as "nailers" for the decking is asking for trouble unless you are willing to use hangers on every block in order to keep them from moving with use of the deck. If you simply nail the blocking, eventually they will settle and the decking will follow the blocks leaving a roller coaster deck surface. I think you would be better to run girders every 4',the girders attached to the planned 2x12's and ledger, and joist opposite/inbetween using hangers attached to the "girts". The girders can be doubled up joist material same size or larger,(2x12's same as your header/ledger).

We rebuilt a porch on our small limestone farmhouse that we bought from a 150 year old house. Our house has slate blue steel roof and we are desperate to make a decision on the colour to pick. We don't know weather to match the exsiting, go with galvanized or go with another color. We are going to use a hidden fastener system. Any advice would be accepted.

I think that older farmhouses often had smaller posts on porches - i.e. just enough to hold up the roof. I like the smaller posts - they work, they are not wasteful, and din't block the views. They also allow you to double up the 2 on either side of the front door. I like the simple, square, plain, unadorned columns and they look nice on the rendering.

I would consider closing in the ceiling rafters and putting in a row of outdoor-rated recessed lighting. You can keep the rustic look by putting in planks that match the rest of the wood. Replace the wall sconces with larger fixtures and add more of them between the windows, too; those look dwarfed in the context of the rest of the space. (These examples are not exactly analogous to your space, but I think you get the idea.)