Okay, so a bit of a Japanese lesson:
'B-gata' would normally refer to the blood type. The Japanese are pretty fond of associating blood types with certain personality traits; and a B-type person is considered to be a bit impulsive, passionate, but their actions 'don't come across clearly' to others. (That seems to describe Yamada's character rather well, doesn't it?)
On the other hand, 'H-kei' normally has to do with 'ecchi' (the Japanese word for 'sex' that came from the pronunciation of the letter 'H' that is an abbreviation for the word 'Hentai'. Phew.) and 'kei' refers to a type (or even a lineage) of something.
Generally put, 'kei' (系) is a classifier for objects (living or non-living) that comes from a tree kind of structure, while 'gata' (型) literally means 'mold' or 'model' and looks at a classification of objects into categories.
All this being said, the official meaning of B-gata H-kei is that the 'B' stands for 'Bousou' (暴走), which can be translated to a 'rampage'; and the 'H' stands for 'Hentai' (変態), which we know is the word for perverted behavior. So the title refers to how Yamada is a rampaging sex-crazy pervert.
I tell you all these pieces of information because they form a sort of pun. The second explanation is the 'correct' one of sorts (in that it's official), but that does not negate the pun that exists in the title - whether it was intentional or not. There is reason to believe the 'B' could also refer to bra-size, as many people have pointed out. That adds to the pun as well, perhaps.
Anyway, I hope this sheds some light on the meaning of 'B型H系'. I can't post too many links here, but check out beta.jisho.org to look up the meanings for the Japanese words I used. Also, this is where I got the 'official' explanation of the title:
It's all in Japanese. The person who had answered that question wrote that this explanation wasn't mentioned in the manga, but in a weekly magazine called Young Jump.
Also, this link: http://detail.chiebukuro.yahoo.co.jp/qa/question_detail/q1139264431
The answer there talks about how the blood types of all characters in the story are actually never mentioned (as of the time of that answer), though the fact that Yamada is a B-cup is known. It also mentions that Yamada as a character is modeled after Sanri Youko (the mangaka) themselves and so speculates that Yamada's blood type could be the same as Sanri-sensei's. (Unfortunately, their blood type is also unknown.)
I'm not a native Japanese speaker (just an enthusiastic learner!) so I might not be entirely correct. But I hope this answer helps, somewhat. Anime titles often have many hidden meanings in them, and it's fun to look for all their possible interpretations.