Color blue is coldest of all colors and relatively rare in living nature. It is color of water or sky, both really being transparent or translucent, somehow out of this world. Man with blue beard doesn't give feeling of a human being. Role of mass murderer suits him well. Blue is also color of authority, often used as the color of police uniforms and similar representatives.
It is color of trust and loyalty, both being tested as the focus of the story. When the trust is betrayed, it is hurt, what can explain Bluebeard's reactions. On the negative side, blue can also represent coldness and lack of emotions, both characteristics of the Bluebeard.
Beard is traditional sign of age, maturity and wisdom. On the other hand it is related to barbarism, magic and weirdness. Bluebeard posses all of these characteristics and more, all further emphasized with unnatural color. A beard can reveal one's status, but also hide his identity, again all true in this case. One of the primal functions of the beard in human culture is to distinguish men from women and if we think a bit, the story of Bluebeard is actually a very polarized contrast between both genders.
All hair is associated with magical powers, what is seen in literature as well. Typical examples are Rapunzel in fairy tales and Samson in Bible. In our case the beard is used only at the beginning, when the title character is introduced, but not later, when his facial hair could probably play more important role. However illustrators used it to full potential.
A key is one of the strongest symbols in literature. It can represent many things. Power is only one of them. Key is definitely important tool and sign of maturity. Kids don't have keys. They don't have property to protect either. And they don't have secrets which need to be locked up!
Keys to the city are handed only to the most people with very special status. Actually every single key can contribute to display of one's status. More keys you have, more important you probably are. There are keys to mysteries and if we are lucky enough (key in several cultures symbolizes good luck as well), we can have a key to one's heart.
Bluebeard used his keys as ultimate test of his wife. Can he trust her? Of course not, in environments with high levels of trust there is no need to lock the doors anyways!
We can't neglect Freud's view on keys either. To him a key is obvious phallic symbol. It represents knowledge by which a girl is distinguished from a woman. Can she handle it? According to Bluebeard, obviously not.