An aqueous polymer blend composition containing particles of a first polymer and particles of a second polymer with preferred molecular weight ranges is provided. Coatings prepared from the aqueous polymer blend composition may be employed as coatings which have water whitening resistance and provide resistance to efflorescence to cementitious substrates, such as roof tiles. A method of preparing a coated cementitious substrate with the aqueous polymer blend composition and an article containing the coated cementitious substrate are also provided.