Slide the prop or hub onto the shaft again and seat it like you did when making the mark. Once seated lock the shaft and spin the hub around it about five-ten times and remove the hub or prop.
There are a number of techniques to using Prussian Blue, I find this one works well but some prefer other methods. The only critical thing is that your method results in a good fit.
In this photo you can see a clean area at the leading edge of the taper where the Prussian Blue has been wiped off. This clean area is the only area this new prop and shaft made good contact. This represents less than 15% contact and even with the prop nuts tight you might have only ever seen 20% contact. This can lead to point loading the shaft and a possible shaft fracture.
This shaft was brand new and made by a shop that does military shafting and has the calibrated dies to test the taper (not all shops have or use these calibrated test dies and they are normally only used to make sure the machines are in tolerance). This taper was tested, with Blue, and was spot on. That leaves the Flex-O-Fold prop taper as the likely culprit here.
This type of "fit" is not uncommon. The sad part is that most yards never even bother to check the fit and instead just slap the prop on. Fits like this are common because machines wear and differ slightly. The prop was machined at one shop, in another country, and the shaft at another.