Yes.
The darkening is from the poulticing.
It will go away as it dries.
Prior to deciding on which poultice to use its best to try and determine what has gotten in to the stone to stain it.
In the case of white marble which can contain ferrous minerals, stains that are yellow in nature can be a result of oxidation.
The stain can be light yellow to deep brown in color.
Oxidation can be tough to remove-iron out (found at hardware and boxstores) can work well sometimes. The problem is the oxidation will return if there is moisture in the equation.
If the client has extra tiles an option to replace them can be considered but if its a spot that is prone to intrusion of moisture then a change out may rust as well.