Black Bottom Pie in SGV?
It appears to be a single-crust pie where a half of a simple custard is mixed with melted chocolate and poured over the flat surface of the crust (thus creating the black bottom), and the second half of the custard is folded with egg whites into a chiffon, which goes over the chocolate-custard mixture. Sometimes the chiffon is rum flavored.
You have it right description-wise. The term is used generically for any pie featuring a chocolate custard topped by a lighter custard; I've seen them with vanilla, banana cream and coconut variants. The generic vanilla-over-chocolate is the most common but also, quite frankly, the most boring.
I have no clue about pie shops in the SGV but I have seen black bottom pies in three places in the LA area: Du Par's, Four and Twenty, and House of Pies.
Wherever you go, call in advance. Few pie shops have every type of pie available every day; you might be able to convince them to put it on the menu that day for you.
House of Pies
1869 N Vermont Ave, Los Angeles, CA 90027
6333 W 3rd St, Los Angeles, CA 90036
Four 'n 20 Restaurant & Pie
5530 Van Nuys Blvd, Sherman Oaks, CA 91401
Four'n 20 Restaurant Grill
4723 Laurel Canyon Blvd, Valley Village, CA 91607
DuPar's Restaurants & Bakery - duplicate
12036 Ventura Blvd, Los Angeles, CA 91604