Before an update in April 2010, black full helms were relatively hard to come by, with the Grand Exchange price generally staying at or near the maximum value. That update added several creatures that dropped them to what had previously been a short list, increasing the supply and lowering the price.
Gold-trimmed black full helms (or Black full helm(g)) and Trimmed black full helms (or Black full helm(t)) can both be obtained as rewards from level 1 Treasure Trails. Like all Black armour equipment, they cannot be made through the Smithing skill.
After black armour was visually reworked, the horns on the black full helm resemble the symbol of Zamorak