Recent years have witnessed the trend of increasingly relying on distributed infrastructures. This increased the number of reported incidents of security breaches compromising users' privacy, where third parties massively collect, process and manage users' personal data. Towards these security and privacy challenges, we combine hierarchical identity based cryptographic mechanisms with emerging blockchain infrastructures and propose a blockchain-based data usage auditing architecture ensuring availability and accountability in a privacy-preserving fashion. Our approach relies on the use of auditable contracts deployed in blockchain infrastructures. Thus, it offers transparent and controlled data access, sharing and processing, so that unauthorized users or untrusted servers cannot process data without client's authorization. Moreover, based on cryptographic mechanisms, our solution preserves privacy of data owners and ensures secrecy for shared data with multiple service providers. It also provides auditing authorities with tamper-proof evidences for data usage compliance