The focus of any sustainable reform of the taxation structure must be on reducing flows of tax evasion, not going after existing caches of black money
The flow of tax evasion every year is what should be the focus of any drive targeting black money, rather than the caches accumulated from past tax evasion. Those stocks certainly existed, but the really large holders of these assets had long ago moved on from physical forms (like cash or gold or real estate) to financial assets. These typically took the form of loans to large commercial construction companies, which simply would not have been able to access the financial capital they needed without borrowing in cash on the informal market. Cash came in particularly handy to pay the wages of construction workers.