Blockchain adds neither things you mention. Both can be accomplished with digital signatures and timestamping, which are not new. Blockchain alone doesn't prev

Blockchain adds neither things you mention. Both can be accomplished with digital signatures and timestamping, which are not new. Blockchain alone doesn't prevent you from modifying data -- just rewrite the entire chain. As an example, Bitcoin only prevents this with proof of work, but only following accumulated difficulty prevents someone from thinking my privately-mined chain is true.