I would be interested in the same thing. Only options i have found would be:
Seachem Onxy sand/Seachem gray coast: I believe these are the same thing. Tom Barr does recommend these for his 'no co2' method. Maybe if used in conjunction with dirt it wouldn't buffer pH up too much.
Oil Dri/Safe T Sorb: pretty neutral on PH. But not as dark as i would like. Also pretty light weight so might cause some issues for planting.
I have wondered about using small granite chips (often sold as chicken grit or poultry grit at farm stores) but i don't know if it can affect pH and it can often be variable in color. I use in container plant substrates. Sometimes its a nice dark gray, sometimes light gray, sometimes reddish.
20g high and 10g planted low tech tanks.