Why are most helipads in São Paulo blue coated and identified by a "P"?
According to the Rules for Heliports in Sao Paulo, the P means that it is private heliport. H for public Heliports,  M is for military. Hospitals use the letter H on a red cross, also when they are private or military hospitals. Examples are given in the pictures below.
It seems that the color blue is not required by the regulations; there are various other colors used in Sao Paulo (see a few examples below I found on Google maps).
I think blue is used mostly because it provide good visual contrast with the white or yellow markings. White paint on concrete can be difficult to see under bright sunlight conditions. The blue color also stands out from the urban environment, making the helipads easy to spot from the air. Try it on Google maps; the blue helipads are relatively easy to find on the satellite view.
This video explains the markings (in Portuguese)

                
            


Hospital helipad with H marking and red cross


Military helipad with M marking


Green helipad with circular border and 05 Marking


Concrete helipad with circular border and 05 Marking


Red private helipad


Green private helipad
All images are screenshots from Google maps