(Note: My answer is based on the assumption that "Blockchain = Bitcoin Blockchain." This does not apply to file block strings such as SiaCoin or Storj)
It depends on how you define "scalability" and "centralized".
If you mean
centralized = on a large computer
Yes, the blockchain could be more scalable, as it can be fragmented: Here is the huge section of frequently asked questions about the fragmented Ethereum.
centralized = many computers in a large company
Relational databases can also be fragmented, so scalability depends on the type of data.
Type of data
Immutable data / data with version tracking.
Blockchain may or may not be a better option, it depends on the implementation.
It is not a good idea to use an immutable blockchain to keep the data mutable. You can simply push the changes to the block chain. Instead, you will have to scan the blockchain from start to finish if you need to access a file, unless the current files are so small that you can save a copy. (<- Which are larger, the files or the version history of the files? It gives shape to how scalability is defined).