Abra o painel do blogger e vá em: Layout > Adicionar Gadget > HTML/JavaScript
Coloque o código abaixo no HTML criado:

<center> <script type="text/javascript"> function showpostcount(json) { document.write(parseInt(json.feed.openSearch$totalResults.$t,10)); }</script><div style="width:108px; height:43px;margin-bottom:5px; position:relative;font:12px<br> Arial,Verdana, Helvetica, sans-serif;"><span style="position: absolute; top: 4px; right: 15px;"><script src="/feeds/posts/default?alt=json-in-script&callback=showpostcount"></script></span><span style="position: absolute; bottom: 6px; right: 28px;"><script src="/feeds/comments/default?alt=json-in-script&callback=showpostcount"></script></span><img src="https://dl.dropbox.com/u/6009047/count.png" /></div> </center>
e Salve.