With the updates of the new patch 7.3, crafting profession changes a lot, which includes the Primal Obliterum.
Primal Obliterum is a new crafting upgrades material, which can upgrade the crafted gears from original ilvl 885 to ilvl935, 5 ilvl upgraded each time, 10 times in total.
The difference between Obliterum and Primal Obliterum is as below:
Obliterum can only upgrade the crafted pre-7.3 Gears to ilvl900, and each upgrade requires 1 Obliterum and 2 Blood of Sargeras. You can't use Primal Obliterum upgrade it up to ilvl 935.
Primal Obliterum can only upgrade the newly crafted gears added in Patch 7.3. The maximum ilvl is 935, and each upgrade requires 1 Primal Obliterum and 1 Primal Sargerite.
Primal Sargerite is similar to Blood of Sargeras, which can be gathered by gathering profession and world quest reward. It is the necessary material to make the newly added BOE gears. Unfortunately, it cannot be traded in game.
The good news is Primal Sargerite can be purchased in AH by wow gold. When the new patch is not coming, the ilvl935 gears still has a great competitiveness.