Multiblock structured mesh generation is amongst the most popular meshing techniques for flow simulations. However, the time consuming and user intensive process of blocking complex domains presents a major bottleneck in structured mesh generation. To this end, we have considered various automatic blocking strategies where some typical aeroengine domains have been showcased. An adjoint based error analysis procedure is then used to assess these methods. It is found that, in general, the medial axis based approaches provide optimal blocking and yields better accuracy in computing the functional of interest. This is because the medial axis based methods produce meshes which have better flow alignment especially in case of internal flows. A new hybrid blocking approach, combining the existing methods with the distance field isosurface is also presented to overcome the shortcomings of the current methods. Such a blocking technique has the potential to be applied to a wide range of flow domains.