I just noticed that I was fairly close to getting Flying Blue Silver (due to the amount of flights flown rather than miles) and wondered if I could use my GF's flights to top up mine.
No, Flying Blue will not permit you to do this. It has to be the member only earning the flights. Sadly your GF's flights will not be able to count towards you getting Silver status.
I only fly once every month or so on KLM so don't really fly often enough to earn any meaningful points
If this is return trips, then you will earn enough flights to get Silver. Currently it is 15 flights (not round trips) to get Silver.
Still, no idea what the difference between Blue and Silver is
Silver doesn't really give a huge amount of benefit. You don't get lounge access until Gold. However, it does give you extra luggage allowance, you will earn bonus miles on all eligible flights, and you can use the priority check-in desks and Fast Track at certain airports. That alone is enough to make it worth getting Silver if you can.
Occasionally you may get a free upgrade. I have had several, although most of my upgrades were as a Gold FF.