How do delta wing or flying wing aircraft maintain their pitch attitude in the absence of an elevator? I understand that the ailerons on the wings do control the banking/roll, but they are differential in nature. Then how can they help in pitch control? For pitch control, both control surfaces need to deflect in the same direction.
You basically have the answer right inside of the question (as was mentioned in the comments). In order to make the plane pitch up and down you have the ailerons move up or down in unison (usually called an elevon at that point, again, as noted in comments.)
Here's a handy diagram to give you some idea of what we're on about:
In the second image the movement of both elevons upwards will pitch the aircraft up as well. And the elevons going in opposite directions, like the last image, will cause the plane to roll (in this case to the left.)
There are more complex maneuvers as well, like pitching and rolling at the same time. These maneuvers are simply managed by differential deflections of the elevons. Meaning, if you want to come up and to the left you could leave the right elevon neutral(ish) and pitch the left elevon upwards. This would push down the left wing, rolling you to the left, and pitch the nose up as well.
The reason an elevator can cause pitch up/down is by providing a force, pushing down/up, on a point aft of the Center of Gravity (CG).
A canard provides pitch up/down by pushing the nose up/down (forward of CG). An elevator provides pitch up/down by pushing the tail down/up (note that pushing the tail DOWN pushes the nose UP).
Are your elevons (combination elevator/aileron) aft of the CG? If so, pushing both of them down will push the tail up and push the nose down.