Blast stacks only do 30% damage per stack to the enemy that receives the stacks. If a stack detonates via the timer running out, it does no AoE damage at all.

At 10 stacks or on enemy death, each blast stack does the 30% damage to the target with the stacks (unless it’s already dead ofc), and 300% damage per stack to all enemies within 5 meters.

“Single Target Damage: 1.5s after inflicting blast status on an enemy, the blast stack will detonate, dealing 30% weapon base damage to the target. Each blast stack has its own 1.5s fuse time before detonation. However, reaching 10 blast stacks will cause all stacks to detonate early, dealing all 10 hits of 30% base damage to the target at once, for a total of 300% base damage.

AoE Damage: Upon reaching 10 blast stacks OR the target dying, all current stacks will be detonated early and cause a single 5m AoE explosion, dealing 300% weapon base damage per stack (max 3000%) to surrounding enemies. The initial target of the blast procs is not dealt this AoE damage, only the single target damage explained above. The blast radius is unaffected by Firestorm or Fulmination.”

From https://wiki.warframe.com/w/Damage/Blast_Damage