Blockchain introduced the Idea of Decentralisation of processing power, trust etc. And the advancements in blockchain technologies like introduction of smart contracts and smart assets over blockchain challenges the very basis of software engineering as distributed applications developed on top of blockchain technologies do not tend to follow rules which are used to make the normal centralised applications. So Why is it so that normal software engineering methods and tools will not apply on blockchain based apps?

Answer is simple - Purpose of both models of development are different.

The Software engineering method like waterfall, agile etc may not fit in aptly for decentralised application development as the these traditional models are with a assumption that all the pieces of all pieces of technology are flexible and can be altered at will (i.e Databases, code etc). But blockchain is a different animal all together once committed to chain no one can change it (Unless the integrity of the blockchain is compromised by 51% attack which is unhealthy affair). Hence Demming cycle / PDCA Cycle cant be applied on Dapp development (Applies both to Smart contracts and smart assets based Dapps.) I do agree normal software development methods would work developing underlying framework (i.e. - Blockchain Clients like bitcoin, ethereum etc) but developing application on blockchains require a different approach. So far no formal framework for decentralised application framework there, but some method which can be used for Dapps development is as follows -

Value Maps

The Value maps is the basic structure of the blockchains and how value flows within the distributed ledger. It forms the very basic layout of the decentralised application and explains how the associated stake holder derives value of form the Dapps. This forms a very high level definition for the Dapps.

Value Flow

Value flow describes how stake holder interact with the blockchain and what do they interact with the blockchain and for certain cases interaction amongst smart contracts as complex Dapps using smart contracts will require interaction amongst smart contracts.

Contract structure

These define the very nature of the nature of the smart contract. It defines the nature of the contract which is working and its interaction with stake holder or other contracts.

As a concluding note, World of blockchains is moving a tremendous pace and will bring in many structural changes to the very fabric we live in.