In a small non-reactive bowl (glass or stainless steel are best), combine the sliced (or cut) scallops and the grapefruit juice. Cover and refrigerate for about 45 minutes while you're putting together the remaining ingredients.
If using the Oaxacan pasilla chile, toast it on both sides in a small ungreased skillet over medium heat until the kitchen fills with its smoky aroma. Cover with hot tap water and allow to rehydrate for about 20 minutes. If using canned chipotle chiles, simply remove them from their canning liquid and place in a blender.
In a small skillet over medium heat, roast the garlic, turning regularly, until soft and blotchy black in spots, 10 to 15 minutes. Cool and peel.
Drain the scallops; measure 2/3 cup of the juice and discard the rest.
Drain the Oaxacan pasilla chile and place in the blender, along with the garlic, piloncillo (or brown sugar), reserved grapefruit juice marinade and 1 teaspoon salt. Blend until smooth. Pour in a bowl.
To the flavoring, add the fruit, onion and optional jicama. Stir in the scallops. Taste and season with salt, usually about 1/2 teaspoon. You can refrigerate your ceviche for an hour or so before serving, or scoop it into small dishes or martini glasses and enjoy right away.