I've never seen the blue RT stripes on any blue cars in person, only in pics and they were all the softer blues. I've never seen the bright 76 b-5 Jamaican blue car with blue stripes in pics or in person, so they had to be pretty rare.
Here's the color charts for 76 and 77 Dodge and b-5 changes colors between the years. They didn't offer b-5 in 78-79
The car you posted and the second one I posted could be either the 76 b4 Big Sky blue or the 77 b5 French Racing blue or 77 b2 Wedgewood blue. It's hard to tell from pics but it's not the bright blue I associate with b-5 like the car in the first pic I posted, which was a '76 only color.
Here's a pic of b-5 French Racing blue:
And here's another pic of a Blue RT with blue stripes
Again, hard to tell which color it is exactly in these pics but it's not the bright 76 b5 Jamaican Blue. And since the car has t-tops, which weren't available in '76, it has to be one of the softer blues in 77. Hope all this makes some sense.