First: the words 'blog' and 'blogger' grate on me. I realise that these are contractions of 'weblog' and 'weblogger', and we are stuck with them, but I don't like them. For me they are words with an unattractive vibe, but no doubt I shall learn to love them - in time.
Mercury must be the foremost influence in blogging, as in any form of writing and communication in general. We all have Mercury somewhere in our charts, but those with Mercury aspected by Saturn and/or positioned close to one of the angles would be more inclined to enjoy writing. Mercury will come through loud and clear in Gemini, Virgo, or in 3rd, 11th, or 7th houses where he feels "at home" but that in itself might not incline a person to writing, especially writing a blog. There is need for a certain amount of Saturnian discipline to sit at the computer and write on a regular basis, with no incentive other than the love of doing it, or the satisfaction of receiving comments. In some cases carrying adverts might offer monetary gain, but I'd guess this is not substantial, except perhaps for a small minority of expert bloggers.
All blog subjects and styles will have their own astrological influence, in addition to Mercury's and Saturn's part in the picture. Politics, the arts and entertainment, religion, travel, pictorial, family, straightforward diary entries about life in general - all will have received a kick-start from some astrological factor in the blogger's chart.
Does this theory work in my own case? I have Mercury in Capricorn (Saturn's influence) and close to the descendant angle giving the planet strength. So far, so good.
I felt most inclined to write about astrology, in spite of my admitted lack of expertise in the subject. Sun in 8th house can incline one towards occult interests (astrology is not the occult, but it's in the same ballpark). Mercury in my chart is trine Neptune and Uranus.
Neptune inspires the imagination, Uranus is said to have links with astrology, so writing about astrology was a good choice for me ! I try to integrate art, authors, politics and travel in my blog from time to time. I'm not sure where the interest art comes from, but being a fan of the written word myself, interest in authors is a natural. Politics comes from an Aquarian Sun, and travel from Venus in Sagittarius.
I've made a case for myself. Now I'll try for my husband.
He has written fairly infrequent 'journal pages' over several years, using blog format.
He is a regular contributor to Flikr which is, in a way, a pictorial blog, and he often transfers his exchanges with others there to his Blog pages. His Mercury is in Pisces, Saturn lies only 3 degrees away, so is conjunct. That's HIS Mercury/Saturn link! Those two planets are also conjuct his Sun which had crept into 1*Aries. Writing for him is a slower process than for me. He edits and edits and edits. I write, look over it once, then post, and correct any errors later. His photography and art interests stem from Venus conjunct Uranus in Taurus within 6* of the midheaven angle.
For both of us there is evidence of interest in writing in general as well as our individual topics.
What differentiates the act of writing for one's own amusement, from writing a blog though? Publishing. Only the lucky, rich or very talented are likely to have their words or pictures published for the potential delight (?) of others....unless via blogs.
I remember reading that Jupiter rules publishing, probably due to the expansive nature of that planet = expanding readership.
So - where is Jupiter in our two charts? In my own case Jupiter is in Pisces (its traditional home) and very close to the midheaven angle making it strong - an urge to publish ? In my husband's case it's in Capricorn sextile Saturn and Mercury - linked to the "writing planets".
I'm not sure whether anything I've just written makes any sense at all astrology-wise.
That's the blessing and curse of blogging. There are no boundaries of right and wrong, accurate, inaccurate. Anything goes. For me, blogging is like playing house when I was a little girl. It's "pretend writing" and "pretend astrology". I don't know whether anybody else reads it. If they do, I'm flattered. If they don't - it doesn't matter one bit, I still enjoy the feeling that "they might!"