Why do you want to build a blockchain on Polkadot?
If you search on Google: "Is building a blockchain hard?"
Building a blockchain is not an easy process. You can take a 4 months bootcamp to learn how to build a fullstack web application. However, with blockchain engineering, it requires strong foundational knowledge in computer science, networking and cryptography to build something that is usable by the industry.
There are a lot of domain specific knowledge to build a blockchain
that is beneficial to the industry
Polkadot SDK provides a framework for you to use to build a blockchain easily without caring much about other primitvie knowledge and low-level engineering like cryptography, virtual machine, database...and only need to focus on building what matters to the logic of the blockchain. This is so-called application specific blockchain.
Substrate Client is a game console while the logic built on top of it is
a game catridge that only need to plug and play
And there is another anology I really like is the LEGO bridge analogy:
Building a blockchain with Substrate is like playing LEGO
What you want to build is a complete LEGO house aka a blockchain
And what you have is single solid LEGO bridge aka a Substrate framework
You just simply add these small blue LEGO bridge on top of your Substrate LEGO bridge.
And BAMP! You have a complete blockchain
