Where can I find actual cases where Blockchain (particularly Ethereum Blockchain) was used in a case, or better yet, decided the verdict of a case
The results of this query reflect that that has not happened in U.S. courts yet.
Mentions of Ethereum in those rulings are marginal and essentially unrelated to the blockchain technology on which major cryptocurrencies are implemented. That being said, the notion of smart contracts in Ethereum is quite distant from the disputes decided under contract law.
In Ethereum, "smart contracts" is the term for programs that will execute a transaction in accordance to a predetermined schedule and/or conditions. Its involvement in contract law seemingly might only be in terms of (1) parties not furnishing the requisite keys, thereby frustrating the execution of that transaction, or (2) the implementation of those programs departing from what was agreed upon between the parties. Neither of these aspects has anything to do with the novelties spoken of Blockchain.
Moreover, the expertise that programming and Blockchain entail implies that verdicts (or more precisely, the fact-finding therefor) would be premised on testimony provided by an expert witness. The average juror is unlikely to have the knowledge and skills set to assess the evidence (e.g. Blockchain, smart contracts, etc.) more directly.