In a large bowl, whisk together eggs, yogurt, milk, vanilla, and maple syrup together. Set aside.
Cut bread into strips and layer half into a 9x13 pan.
Scatter 1 cup frozen blackberries throughout bread, then layer remaining bread, and top with remaining frozen blackberries.
Pour custard over top. Allow to soak in; refrigerate at least 30 minutes to an hour.
Preheat oven to 350. Lightly pat down any bread that floated up a bit. Place into oven for 45 minutes to an hour.
Over medium heat, add blackberries and maple syrup and bring to a boil. Allow to simmer for 5-10 minutes.
In a small cup, make a slurry with cornstarch and vanilla. Pour into blackberry maple syrup mixture, bring to a boil for a minute until thickened, and then take off of heat to cool. Store in the refrigerator for 4-7 days. If it lasts that long.
Some loafs are smaller than others. If you loaf is on the smaller size you may need to use less of the custard mixture.