Buy an iPhone, iPad or Android, write an app and you're done. Really. There is simply no way to beat the prices or the quality of these. Absolutely no chance.
And the market for blogging is so small compared to even standard SMS or POTS that you'd never make the economics work for custom hardware - unless you want to sell them for $1K-$3K each.
This is because you already have to implement everything that existing smart phones implement just to connect to the internet. And then your market is smaller, so demand is lower (and economically viable prices must be higher)