Why do pregnant women turn toilet seats blue?
The question cannot be answered without a chemical analysis.

However, given that there exist toilet seats with silver based anti-microbial coatings, the bluish grey color may come from metallic silver particles produced by a reduction process similar to the photographic development process.

You could proof the presence of silver by applying Caffenol, an alkaline aqueous solution of instant coffee, washing soda, and vitamin C (ascorbic acid) to the toilet seat. I suggest to use Caffenol-C-M whose preparation is described here. Let the Caffenol react on the toilet seat as long as possible. If grey or black stains are formed that persist after cleaning, your toilet seat contains silver with high probability. Note that the stains cannot be removed anymore.

If the test is positive, the next step would be to investigate why your pregnant partner has reducing powers.