Blue jets and blue starters are considered as positive streamer coronas expanding from the streamer zones of conventional lightning leaders under conditions when large-scale electric fields near the thundercloud tops exceed the minimum field required for the propagation of positive streamers in air. Results from a three-dimensional fractal model based on a phenomenological probabilistic approach to the modeling of streamer coronas indicate, in particular, that blue jets and blue starters can be formed by a fast (~1 s) accumulation of ~110--150 C of positive thundercloud charge distributed in a volume with effective radius ~3 km near the cloud top at ~15 km. The model simulates the propagation of branching streamer channels constituting blue jets and blue starters as a three-dimensional growth of fractal trees in the electric field created by thundercloud charges and self-consistently accounts for the electric field effects due to the propagating streamers. Model results closely resemble blue jet and blue starter characteristics in terms of their altitude extents, transverse dimensions, and conical structure and indicate that blue starters are related to the initial phases of blue jets. The proposed model is supported by the recent spectroscopic observations of blue jets and blue starters, in particular, by the documentation of the 427.8 nm (first negative N2+) emission in blue starters as well as by the low-pressure laboratory experiments on emission spectroscopy of corona streamers in air. The model results also appear to be in excellent agreement with the recent discovery of the streamer structure of blue jets.