What is the brightest color a light source that only emits blue frequency light can achieve?
The color that human vision perceives for a specific frequency of light (prior to saturation of any cones) is derived from the relative responses of different cones.  For example, at about 480nm, the response of S and M cones are roughly equal.  However, the human eye is not able to distinguish pure colors (single wavelengths) from combinations of wavelengths which produce the same response in each of the three cone types.

It is possible that a blue light source could appear white to the human eye, because the cones which inform our perception of color are not strictly monochromatic.  Rather, they have a range of sensitivity across the spectrum, and all cones are at least somewhat sensitive to blue light.  (See this diagram.)
If the blue light is so intense that it saturates all cones, S (blue), M (green), and L (red), then it will be perceived as white light.  However, in order to saturate the low-sensitivity M and L cones, the intensity of the light would be blindingly high.  Prior to this point, but after saturating the S cones, the apparent color of the light would shift toward green before eventually appearing white.  For intensities that are not saturating to any cones, the hue of the color blue will remain constant, while it's value (or lightness) may change.