(Note: My answer is under the assumption that "Blockchain = Bitcoin Blockchain." This doesn't apply to file blockchains such as SiaCoin or Storj)

It depends on how you define "scalability" and "centralized".

Centralization

If you mean

centralized = on one large computer

Yes, the blockchain might be more scalable, as it can be sharded: Here's the huge Ethereum sharding FAQ.

centralized = many computers in a large company

Relational databases can also be sharded, thus scalability depends on the type of the data.

 Data type

Immutable data / data with version-tracking

Blockchain might or might not be a better option, depends on the implementation.

Mutable data

It's not a good idea to use an immutable blockchain to keep mutable data. You can just push the changes to the blockchain. On the contrary, you will have to scan the blockchain from start to end if you need to access one file, unless the current files are so small that you can keep a copy of it. (<- Which are bigger, the files or the version history of files? It shapes how you define scalability.)