// Outside of any functions...
double lastBlinkTime;
static const double blinkInterval = 500;

// A function that is called once at the start of the game...
void Init()
{
    lastBlinkTime = 0.0; // Or you can use the current time, doesn't matter really at this point
}

// A function that is called every frame
void Update()
{
    // Here you should use whatever you have available to get the current time in ms
    double curTime = getCurrentTimeInMilliseconds();

    if(curTime > lastBlinkTime + blinkInterval)
    {
        lastBlinkTime = curTime;

        // Do whatever blinking code you need to here.
    }
}