Trying to measure change in light levels over time
Break this into pieces. Solve the pieces one at a time. 

Write code that reads the 2 photocells and calculates the difference between them. Log that difference to the console. 

Then add a global variable to remember the old difference, and a max difference. Each time you take a new set of readings and calculate the difference, compare it to the old difference. If the new difference is greater than the old difference, update the max difference. Then update the "old difference" with the new reading.

Figure out how to use millis() to record a starting time, and monitor the reading difference until an hour has passed. If the reading difference is below your "large amount" threshold, start the light blinking.

Then you need to decide what to do about times greater than an hour. Do you track the max difference over the most recent 60 minutes and flash the light if it is below your threshold? Maybe save a difference reading every 5 minutes into a ring buffer with 12 entries (an hour's worth of entries) and keep checking the ring buffer for a difference value greater than your threshold? (You should be able to Google search "C ring buffer" for sample ring buffer code. It's actually quite simple to implement.

You might also want to calculate a moving average difference reading, and compare each reading to that. I recommend using an Exponential Moving Average (EMA). 

I have code in my project that uses EMA to calculate a single brightness value rather than a difference, but the idea is the same. Here is an excerpt from my code:

Define a global:

float   photocellAverage =      0;

#define sampleCountToAverage 100  //The larger the number of samples, the more slowly the average will change with each new reading.


Then 

int photocellValue = analogRead(photocellPin);
photocellAverage -= photocellAverage / sampleCountToAverage;
photocellAverage += float(photocellValue)/sampleCountToAverage;


For you, you'd be calculating a difference reading and an EMA of that difference reading:

float   photocellDifferenceAverage =      0;

int photocell1Value = analogRead(photocell1Pin);
int photocell2Value = analogRead(photocell2Pin);
int photocellDifference = photocell1Value - photocell2Value;
photocellDifferenceAverage -= photocellDifferenceAverage / sampleCountToAverage;
photocellDifferenceAverage += float(photocellDifference)/sampleCountToAverage;