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What it does
A cyanotype is not a blue tint over a photograph. It is a single-channel process: the original colour is discarded entirely and tone alone decides the result. Blueprint works the same way, which is why it looks like a print rather than a filter.
How it works
Colour is reduced to perceptual luminance first, so the mapping responds to how light something looks rather than to its hue.
Contrast is applied with a slight upward bias, because the historical process is contrastier than a straight photographic curve.
Tone is then mapped through a two-segment gradient: deep navy into cyan-blue across the shadows, then cyan-blue into a pale paper wash across the highlights. The midpoint hinge is what gives the deep shadows their weight.
A fine per-pixel value grain is added to stand in for uneven exposure across a hand-coated sheet.
What each control changes
Contrast
The most consequential control here, because it decides where the shadow and highlight segments meet the image's tones.
Intensity
How completely the blue mapping replaces the original. The effect is composited over the untouched image, so reducing this reintroduces the original colour as a wash.
Reseed
Redraws the per-pixel exposure grain from a new seed without touching the tonal mapping, which helps when the grain clashes with fine detail.
Ink palette
Cobalt is the archival default; the others move it away from cyanotype toward other monochrome processes.
Good for
Architectural and technical imagery
Botanical and still-life studies, the process's original subjects
Monochrome artwork that needs a historical register
Less suited to: Images that depend on colour to be understood, since hue is discarded before mapping.