IMHO, probably any such app will have only some utility for short term.
It's difficult to evaluate the value of any crypto in the local fiat curency unless crypto-exchanges are legalized. Who will judge what's the value of the crypto-assets in the absence of exchanges? Today, the whole crypto-market is very volatile. Also, there is a huge difference in rates among various crypto-exchanges and the difference increases when country specific crypto-exchanges are taken into account. But everyone is free to trade in his/her exchange of choice. Also, there is a little fluctuation in rates of local fiat vis-a-vis USD which is being used to transact crypto from fiat. So which rates would be considered for taxation purposes?
IMO, in the coming times, advanced privacy features on blockchains will be developed and it won't be easy for anyone to check your activity on that blockchain. So tax authority will need to depend on self-declaration of assets.
In my country, taxation rules say that a citizen needs to pay tax even on income from theft, robbery or any other illegal means. But how many people do that?
http://www.steemreports.com/
Good tool for calculating all earnings from Steem blockchain.