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)
Not intending to dampen the enthusiasm too much with my answer but if you tried to get money for the idea from an Angel or VC, they'd be asking the same question: "why didn't you just...?"
And then they'd ask the really hard questions about your market and financial models, etc. Or worse.
I suppose I should also point out I've co-founded starts-ups in Silicon Valley. Because I have an BSEE and MBA I've usually been the one to do the business plans and market research/plans while others work R&D, so these kinds of questions are bread-and-butter.
When you come up with business ideas you have to be your own worst critic - creativity and enthusiasm without iron self-discipline and self-criticism are generally worthless.
Purely creative is like the Michael Keaton character in the movie http://www.imdb.com/title/tt0084412/" [Broken]:
"Idea for tuna fish salad: feed tuna mayonnaise; call Star Kist in the morning!"
That's actually the point of the movie: Henry Winkler is the self-discipline/self-criticism and only together can either be successful.