Blueberry Grapefruit (Pudding?) Cake

(Photo taken by my S3 #naturallightFTW #androidFTW)

Verdict: win

I have been dreaming of pudding cakes. I recently sampled a blueberry orange pudding loaf cake at the local grocery store (Giant Eagle Market District), and ever since then I have been obsessed with recreating it. It was a light pound cake, incredibly moist to the point that it was falling apart as I would cut a slice, and moderately sweet so I felt like I could eat the whole loaf. It completely changed my outlook on non-chocolate cakes.

The problem is, what exactly is pudding cake? Does the "pudding" refer to its texture or that it contains pudding? I have spent hours trying to understand the definition of a pudding cake, but I can't find any explanation with the recipes of the cakes that seem to resemble the pudding cake I tasted. I even took a picture of the ingredients list to try to decipher it:

All I could make out was that it uses "yellow pudding cake" (again, what is this?), sugar, flour, eggs, skim milk, baking soda, soybean oil, eggs, orange juice, blueberries, and sour cream.

Several recipes suggested "doctoring up" cake mixes, so this is what I tried first. Here is my first effort using yellow cake mix, vanilla instant pudding mix, and sour cream. I subbed in grapefruit (I may have bought 10 lbs of grapefruit recently...) and olive oil. The grapefruit was subtle (the zest only helped with the aroma), so I may add more grapefruit juice next time. I would also recommend a more neutral oil like canola oil, as the olive oil flavor was overpowering. I could still taste the boxed yellow cake flavor and it wasn't as moist as the original, so my next attempt will be a recipe from scratch. Thinking about brushing on a syrup to help with the moistness next time too.

Overall, it was tasty and incredibly easy, but it wasn't perfectly like the original. Then again, I was intensely critical of this cake after dreaming about the original for so long. Either way, I'm still confused if this falls under the definition of a pudding cake. Any suggestions would be much appreciated!

***Update 5:11 PM
Okay, just tasted the center of the cake after letting it cool. Definitely more reminiscent of the evasive GEMD pudding cake. Recipe is a keeper, but still want to try making this from scratch.

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Blueberry Grapefruit (Pudding?) Cake
Yield: one 9x13 inch pan

INGREDIENTS
1 box yellow cake mix
1 box vanilla instant pudding (4 serving size)
4 eggs
1/2 cup grapefruit juice (about 1 grapefruit)
1/2 cup water
1/3 cup canola oil
1 cup sour cream
Zest of 1 grape fruit (optional)
8 oz frozen blueberries

DIRECTIONS
Preheat the oven to 350 deg F.

Mix all of the ingredients except the blueberries together in a large bowl. Mix well, but try not to overmix. Pour batter into a buttered 9x13 inch pan. Sprinkle blueberries on top and gently swirl into the batter (allow most of the blueberries to sit on top, because they will sink toward the bottom while baking anyway).

Bake for approximately 1 hour or until a toothpick comes out clean from the center.