Both are S-tier Faith greatswords obtained from Remembrances. Blasphemous Blade dominates PvE with healing; Sacred Relic Sword dominates farming with Wave of Gold. Which fits your playstyle?
Blasphemous Blade
PvE exploration and bossing
Strengths
- HP-on-kill passive provides incredible sustain
- Taker's Flames has excellent range and damage
- Available mid-game from Rykard
- Fire damage is widely effective
- Works without buffs
Weaknesses
- Fire damage resisted by some bosses
- Less effective for rune farming
- Requires Strength investment alongside Faith
- PvP weapon skill is predictable
VS
Sacred Relic Sword
Rune farming and NG+ clears
Strengths
- Wave of Gold has the widest AoE in the game
- Best rune farming weapon at Mohgwyn Palace
- Excellent for NG+ speed clearing
- Dexterity/Faith scaling is unique
Weaknesses
- Only available after final boss
- Holy damage resisted by many endgame bosses
- No sustain mechanic
- Requires completing the game first
Verdict
Blasphemous Blade wins for a single playthrough — it's available mid-game and provides incredible sustain. Sacred Relic Sword wins for NG+ and farming. Ideally, get Blasphemous Blade first, then Sacred Relic Sword for subsequent cycles.