A Dihedral is simply 2 converging angles. What this means when related to paddle design is by incorporating 2 slight angles on the powerface we can direct the water to flow evenly to each side of the paddle, which reduces or eliminates flutter depending on the efficiency of the angles.
Think of the bow of your canoe. It has a center line with 2 symmetrical sides to 'split' the water and push through it keeping you in a straight line. If the bow was square as a brick you can imagine the difficulty you would have in tracking.
We obviously don't want a paddle as angled as a bow, but the concept is the same. Using a very low angle we can still provide plenty of blade catch but gain the advantage of reduced flutter and an easier, less tiring stroke.
Now comes the obvious question, If a dihedral offers an advantage why dosen't every paddle have one? As with most things in life, the answer is cost. It takes more time, tooling, and trained workers to make the angles, so most manufactures simply don't do it. You see it more often on synthetic paddles where using a mold dosen't increase any manufaturing costs since the mold is only made once.
Since you are making one paddle that will be yours time and effort are not a major issue, what's an extra 30 minutes of shaping in the long run.