Complete the game to unlock concept art on the Main Menu.
Boy and His Blob easter egg:
On the final level "escape", just after crossing both lava pits, if you high jump up the left wall you can climb it. Continue climbing (watch for bullets shooting you) and dash right when you make it to the top. You can't turn into the raven due to the machines so dash right as much as possible until you find the red skull. At the final wall, wall bounce over it and you will find the two cute characters behind bars.
Press the following all at the same time to unlock 8-bit soundtracks: L1, R1, L2 R, L3, R3, START and SELECT.
Chainsplosion! - Chain-explode 5 enemies
Coffin Kicker - Kick open the Coffin Rocket 20 times
That Funky Puncher - Defeat Crab Puncher without getting hit
The Last Bullet - Kill 5 enemies with a single gunshot
Walking on Air - Hop on 50 heads in a row without touching the ground
8 Fingers, 8 Bits - Activate BloodRayne: Betrayal's alternate soundtrack
A Foot Face? - Find the hidden idol
Collateral - Kill 250 enemies with environmental hazards
Ms. Perfect - Complete any level while taking no damage
Skulltastic - Collect all the ancient vampire skulls
Survived to Sunrise - Complete the game
Pro Vamp - Finish every level with the ranking 'Dhampir'