window blinds in attic flat

I have a flat in an attic and it has those tilting windows. This means I have to buy special window blinds. It's very hot in the flat, so i must have blinds. Here's the question:
The lady at the shop insists that they should be dark as possible to block all the light. The problem is the heat and i thought white would be better due to higher reflectivity. Anybody with experince?

I guess they don't sell blinds that are tinted or reflective on one side, yet you can still she through them like sunglasses?