Create a blinking label

What can I do to make my Label blink?

Please show what you have tried till now. Even if it may not work as you expect, that would be very useful to write an answer.

I tired this in the viewDidLoad() function, but this does not show me the blinking, instead the View show up, when the for-loop has finished.

Code Block 
	caption.backgroundColor = UIColor.black
        for _ in 0...2{
            var bool = false
                if bool{
                    caption.backgroundColor = UIColor.black
                    bool = false
                }
                else {
                    caption.backgroundColor = UIColor.systemRed
                    bool = true
            }
            sleep(1)
        }

I tired this in the viewDidLoad() function,

It would be better if you could show more context.

There's one very simple principle in iOS programming: NEVER call sleep.
(Never call blocking functions in the main thread.)

All the UI updates begins when the current task in the main thread is finished.
UI updates starts after viewDidLoad() is finished, meaning all the intermediate results are not shown and only the last result done in viewDidLoad() will be shown with very long delay.

As you wrote yourself, using Timer is one of the right ways:

Code Block 
        caption.backgroundColor = .black
        var backgroundColorIsRed = false
        var blinkCount = 0
        Timer.scheduledTimer(withTimeInterval: 1.0, repeats: true) {timer in
            backgroundColorIsRed.toggle()
            self.caption.backgroundColor = backgroundColorIsRed ? .systemRed : .black
            blinkCount += 1
            if blinkCount >= 4 {
                timer.invalidate()
            }
        }

I did this with a UIView extension

Code Block 
extension UIView {  
    func flashIt(repeated: Int, cycleTime: Double, delayed: Double = 5.0) {
        if repeated < 0 { return }
        
        let initAlpha = self.alpha
        UIView.animate(withDuration: cycleTime,  //Time duration
            delay: delayed,
            options:[.allowUserInteraction, .curveEaseInOut, .autoreverse, .repeat],
            animations: {
                UIView.setAnimationRepeatCount(Float(repeated)) // repeat 3 times.
                self.alpha = 0.1    // Not 0.0, to allow user interaction
        },
            completion: { (done: Bool) in self.alpha = initAlpha } )
    }
}

In your case, instead of setting the alpha, just set the background color.
And restore it in completion.

@OOPer thank you very much for you advice and the solution! It works like it should. Now I call a method in the viewDidLoad() called blinkred(), is that correct?

Now I call a method in the viewDidLoad() called blinkred(), is that correct?

It depends on how you implemented blinkred(), but I guess you have done it correctly if you can see the right result.