Venus is transiting today for the last time until 2117 (or 5878, if you prefer). Is there a specific bracha that should be made upon witnessing this spectacular event?
A quick search of discussions on the web seems to point to there being no particular bracha and that Oseh Ma'aseh B'reishit is not called for, for a solar eclipse. As a kal vachomer, I would say that if there is nothing said for the still infrequent but more spectacular solar eclipse, then for the transit, why would there be? This site suggests other textual options at least as it relates to lunar eclipses -- and the kal vachomer could work from there with those psukim being said for other events.
See my answer here regarding a modern dispute whether we can extend the scope of blessings to natural wonders not explicitly listed in the Talmud.
Regarding the transit itself: it would seem to fall into the possible exception I mention there regarding eclipses (where all agree there is no blessing) because when the Talmud discusses the fact that eclipses are a bad omen (Sukkah 29a), it discusses eclipses of the sun, the moon, and the מאורות-[heavenly] lights. Rashi there explicitly includes stars in this. As Venus is the brightest 'star' in the night sky, I submit that an "eclipse of Venus" (ie a transit) would also qualify as a bad omen, and would thus possibly not get a blessing even according to the more lenient opinion in my other answer.
This is just my idea. Please make sure to CYLOR sometime in the next 105 years before relying on anything you see here.
SAH correctly points out in that Venus is eclipsing the sun, not being eclipsed. However, I note that during the transit, Venus appears black against the larger bright sun, not affecting its total brightness (at least to the extent that we can notice). So in terms of which body is considered to be in ליקוי='diminished', it would seem that Venus is the one despite being in front of the sun.