Blue roofs are not intended to provide permanent water storage. The purpose of blue roofing is to temporarily hold rainwater to avoid overwhelming local urban surface water systems. It is then released under controlled conditions back into the systems.
The water can be used, however, to irrigate green roofs when these two technologies are combined. In some cases blue roofs can be harvested, and the water used within the home, for flushing toilets, for instance. This can be subject to restrictions in some areas, so it is always wise to discuss your project with the local planning authority.