My birman is an indoor cat only - a lot of pedigree breeders sell to indoor homes only for many reasons -
1 - the risk of theft of a very expensive pet
2 - pedigrees quite often will walk off with anyone that shows them attention and get lost
3 - risk of a road traiffic accident (cats can wander a long way - my old moggie was shot
5 miles from home by some b******d and dragged herself back (shot 3 times). Birmans certainly seem to have no road sense
4 - the risk of poisoning, thugs who torment cats etc (draw a 10 mile radius around your home and see if anyone else lives in it or any largish road nearby and then you know if you are safe or not)
5 - the risk of disease etc.
6 - territory wars between cats etc
If you are seeing blue cats around they are probably moggies with an element of some pedigree somewhere in the last few generations in order to get the blue (but it can occur without pedigree influence). I once had a blue that was moggy.
You could always build a cat run or make your garden secure.