"Dark angels" or "black angels" depends a great deal on perspective, as others have discussed.
Some traditions carry that the Nephilim were the result of angelic union with humans, creating offspring that was an unholy mixture of both. These were often considered giants, and were said to have been destroyed during the flood in most lore regarding them.
Conversely, other apocryphal traditions suggest that when "Satan" or "Lucifer", again depending entirely on what you are reading, rebelled, a host of angels rebelled with him. These angels were also considered fallen.
Like any spirit, contacting them may yield knowledge and understanding. To my experience, however - which is unverified, and contrary to most other doctrines - no angel is particularly fallen. They are all seeking to do divine will, even "Satan" or the adversary, as divine will is to simply shepard and coexist with the whole of creation. They are, in general terms, powerful "higher" spirits that do not enjoy being called upon for petty reasons, without traditional formalities being observed as a sign of respect. If such courtesies are observed, they make indeed give an oath of servitude so to speak, and offer up services to the individual who calls them. This oath is more the foundation of a bond that any two spirits might possess between one another, and creates a link between them and the one to whom they make the oath as well.
However, no matter the bond or oath created, I rather doubt that they will provide a cheap and easy way to improve one's intellect. There are no shortcuts to this - study and hard work are favored, and angelic presences almost always advocate this first and foremost.
Some demons may offer aid and understanding, and something akin to short cut, but those that are worth calling require similar ritualistic endeavors, and in the end the cost that they demand of you is sometimes painful - but conductive to personal growth.