According to the wiki (https://oldschool.runescape.wiki/w/Dark_essence_block) these blocks are untradeable so each blood/soul rune really does *require* gaining the bit of rc xp for making the dark essence blocks. There is the theoretical situation in which someone just mines and makes dark essence without turning it directly into runes (maybe banking it?) but this seems pretty damn unlikely since that would just be so inefficient compared to turning it into runes directly.