Welcome to Android Central! When you take the phone away from your ear, look at the Dialer screen. Tap the End Call button (which is usually red, and has an icon that looks like a phone on a cradle).
Move your phone from your ear and just tap and hold your finger on the red button and move your finger aside keeping it pressed to the screen. Some of the phones (and blu phone probably too) have an opportunity to end the call by pressing the power button or simply turning it upside down (check the special options of your phone).