It would be nice if a dev of DRK could explain that or at least a DRK user. Why exactly is it closed source? When will this be changed? Why should one trust DRK?
Guys, all the info can be found on darkcointalk.org. Stuff is already working on the testnet. Everything apart from the darksend functionality is open-source already. It will be open-source once it's fully functional on mainnet and there was a full audit.
Suspecting mining to be tampered with is ridiculous. There are open-source miners available and they work as expected. You can look it up on the blockchain anyway.