If you want to fade all the LEDs form blue to red at the same time, I think you want code like this (explanatory comments embedded)...
#include <FastLED.h>
#define DATA_PIN 6
#define NUM_LEDS 7
#define COLOR_ORDER GRB
CRGB leds[NUM_LEDS];
void setup(){
FastLED.addLeds<WS2811, DATA_PIN, GRB>(leds, NUM_LEDS);
}
void loop(){
// Let's take 256 steps to get from blue to red
// (the most possible with an LED with 8 bit RGB values)
for( int colorStep=0; colorStep<256; colorStep++ ) {
int r = colorStep; // Redness starts at zero and goes up to full
int b = 255-colorStep; // Blue starts at full and goes down to zero
int g = 0; // No green needed to go from blue to red
// Now loop though each of the LEDs and set each one to the current color
for(int x = 0; x < NUM_LEDS; x++){
leds[x] = CRGB(r,g,b);
}
}
// Display the colors we just set on the actual LEDs
FastLED.show();
delay(10);
}
}