Geek and geeky have a wide variety of meanings. Until about six or eight years ago the sense was usually negative; but the geeks themselves started wearing the label with pride, and the meaning has shifted,
I think it quite proper to invent a new way of using the word to express a distinctive meaning. This particular use, be geek suggests that you are inviting the reader not to be a geek, a social role to which certain qualities are attributed; certainly not be geeky, to exhibit certain (usually deprecated) behaviors; but to aspire to the noble qualities of geekitude, to release their inner geek.
In any case, it's your blog, and you can say anything you like.