What kind of Java type is "[B"?

As the other answers state, that is a byte array.

If you want to get a string from a byte array, use the String constructor:

public void testCrypto()
{
        session.beginTransaction();
        // creates native SQL query
        // uses native MySQL's MD5 crypto
        final String pass = new String(session.createSQLQuery("SELECT MD5('somePass')")
            .list().get(0));
        session.getTransaction().commit();
}

It's the class name of byte[].class. Try this:

System.out.println(byte[].class.getName());

Output (you guessed it):

[B

And if you want to access the readable name, use Class.getCanonicalName():

System.out.println(byte[].class.getCanonicalName());

Output:

byte[]

[B is the encoded type name for a byte array (byte[]), which should normally only appear in type signature strings, as its not a valid type name.

That my friend is an array of bytes. In JNI, [B is used to describe an array ([) of bytes (B). An array of ints is [I etc.

You can get a bit more information on field descriptors here:
JNI Types and Data Structures (Table 3-2 should be what you are looking for).