I am not quite sure. Let me be more specific.
Dask requires you to encode computations as a graph explicitly, essentially forcing you to write an AST manually. This means that you are sidestepping the normal language mechanisms to encode program flow, and doing so is essentially incompatible with anything else. Moreover, it does not use numpy arrays - it uses its own internal format that you can convert to a numpy array when needed as explained the overview: http://dask.pydata.org/en/latest/array-overview.html
Numba is another cool project, but - again - it deviates from the mainstream Python. Not every Python function is compilable by Numba, and arithmetic is fixed-size. This means that existing Python code may or may not be compilable by Numba, and even if it works one has to carefully check that arbitrary precision arithmetic is not used. So, it is nice when it works, but is little more than a way to write C-like code with a Pythonish syntax (in which case I greatly prefer Nim, that has actual dispatching on types, generics and so on).
Blaze is in a strange position which I do not fully understand. Apparently it uses Numpy arrays, but at the same time the Numpy author states it should be a Numpy replacement: http://technicaldiscovery.blogspot.it/2012/12/passing-torch-...
Numexpr requires you to write your code as strings, so it is essentially a separate interpreter. It is nice that it is fast, but it does not play with normal Python code.
Not a single project of these works on PyPy, which is the only fast interpreter for non-numeric Python code, nor on Jython, which is the only multithreaded Python interpreter.
Do not misunderstand me: I enjoy Python for many things, and internally we use it a lot. But I am frustrated that a lot of effort seems to be dedicated to fix things at the language level starting from a powerful library ecosystem, where I would like to see something solid at the language level that evolves libraries instead