Just what diversity obligations a blog has is a slightly tricky matter. I think anyone is perfectly within their rights to start a solo blog, and if that blog’s authorship is thereby 100% white and male, I don’t see how that’s a problem. I don’t think there’s a problem if they add a second author, even if that still means 100% white and male. A philosophy-oriented group blog that had, say, 10 authors and was 100% white and male, now that I would think was troubling in its lack of diversity. My intuitions about these cases feel fairly strong and robust, and I assume they are tracking something, but I don’t have a good theory about what they are tracking.
Process matters. I think the relevant feature here is not the size of the blog, but rather whether the pool of potential contributors is open-ended. Suppose, for example, a dozen drinking buddies decide to set up a group blog as an online supplement for their in-person philosophical banter. Despite the larger size, such "social group" blogs are relevantly like individual or two-person blogs. Assuming you don't have "diversity obligations" regarding the company you keep, you likewise won't have such obligations regarding the blogs you set up with those friends -- whatever their number might be.
A different kind of "closed" contributor base is found in department blogs, for example. So long as everyone feels welcome to join the blog, there doesn't seem any further moral issue at stake. It may turn out that the global result of respecting each individual's preference (to participate themselves or not) is that disproportionately few women end up blogging. But so long as each individual freely chose what they wanted (without distortion induced by fear of hostility or the like), this procedural fact is surely all that matters. Whatever outcome results from the aggregate of individual choices in this situation is ipso facto just.
Things seem different when we consider an "open-ended group" blog, e.g. topical group blogs that welcome any contributors who work in the relevant field. Because it is open-ended, not every potential contributor receives an explicit invitation to join. And if, of all the experts in the field, one only thinks of men to invite, such implicit bias (however innocent) certainly seems unfortunate -- and worth remedying (e.g. by making an explicit effort to remember women in the pool who are equally deserving of an invite).
So far, this all strikes me as fairly commonsensical (no?). But what if, despite all procedural fairness, it turns out that women are less likely to accept the invitations? Is one obliged to take further, affirmative steps to promote "diversity" in outcome? This is more controversial, as it rehashes the whole 'affirmative action' debate.
On the one hand, it seems plausible that increasing the visibility of (expert) minorities in the field could have good consequences, e.g. in counteracting implicit bias, and perhaps in making the field seem more welcoming to minority students. So there's probably some reason to affirmatively recruit minorities.*
Still, as a deontic minimalist, I don't think such efforts could be reasonably demanded, or considered 'obligatory' as opposed to simply desirable. (Maybe this is even true of the procedural case, above, for explicitly counteracting one's implicit biases?) It'd be a good thing to do, but perhaps not so pressing as to justify burdening individuals with moral demands. Consider: if someone goes to the effort of creating a new and worthwhile group blog, it seems rather perverse to expose them to moral censure for their failure to exert additional feminist efforts in the process. (And writing letters of complaint to the "higher-ups" at affiliated institutions, as I think I saw someone suggest, seems downright counterproductive.)
Moralists should inspire, not harangue -- or so I'm inclined to think, at least. (Perhaps not everyone reacts so negatively to haranguing?)
* = It's worth noting that affirmative action generally risks burdening brilliant women with self-doubts about whether they were selected in part for their gender, and not just on their merits. This seems less of an issue in the particular case of blogs, since they're never all that selective to begin with. But it may be a reason to oppose AA practices in general (especially if the benefits are minimal, and more effective means can be found to make the field inviting to minority students).