The solution processability of a gelating polyurea network has enabled its blending with polymers, allowing facile preparation of bicontinuous nanoporous materials. To exploit the method for synthesizing various functional porous structures, a deeper understanding of the structure-forming mechanism and the relationship between the experimental parameters and the morphology is crucial. Herein, we experimentally show that the thermodynamic parameters of the mixture of the covalent network and polymer govern the formation of initial blend morphology, and control over the covalent network's gelation kinetics under solvent evaporation is critical to obtain the nanoscale structures otherwise difficult to achieve. We first investigated the phase behaviors of various covalent network/polymer mixtures in solutions and the morphologies of their resultant blends and porous structures. The different morphologies were mapped in a single diagram based on the polymer composition and the empirical descriptor devised to include the components' mutual interactions and relative sizes. To capture the phase-separated structures at the nanoscale, we found that the two coexisting phenomena should be controlled: the gelation via interconnection of the covalent network nanogel particles and the growth of phase-separated domains with solvent evaporation. We proposed a general mechanism involving the interplay of gelation and phase separation with an evaporative cue using a ternary phase diagram of the polymer/covalent network/solvent. This work lays the groundwork for designing and optimizing functional porous materials derived from gelating covalent network/polymer mixtures, in addition to establishing an empirical approach in a reactive colloidal polymer/polymer mixture system.