I dont think its a Bloodline Limit. A Bloodline Limit is where a special technique is passed down to each member of a clan. Like the Uchiha's Sharingan or Hyugga's Bkaygun (however you spell that). I think that Tsunade's strength is more into the genetics like with dominant and recessive genes and she happened to most likely had a dominant gene in strength, just like the First Hokage.