You can only activate this card if there are 30 or more cards in your Graveyard. Inflict 3000 points of damage to your opponent's Life Points.
Rulings
Individual Card FAQs
Even if you activate "Blasting the Ruins" and an effect is chained that reduces your Graveyard to less than 30 cards, "Blasting the Ruins" still resolves because the condition was correct at the time of activation.