An extension that allows you to freely adjust the amount of blur on a UIVisualEffectView. Tested on iOS 11.2 with Swift 4

UIVisualEffectViewAdjuster.swift

extension UIVisualEffectView {
    public var adjuster: UIVisualEffectViewAdjuster {
        get {
            if let adjuster = objc_getAssociatedObject( self, &UIVisualEffectViewAdjuster.key) as? UIVisualEffectViewAdjuster {
                return adjuster
            } else {
                let adjuster = UIVisualEffectViewAdjuster(effectView: self)
                objc_setAssociatedObject( self, &UIVisualEffectViewAdjuster.key, adjuster, .OBJC_ASSOCIATION_RETAIN)
                return adjuster
            }
        }
    }
}

public class UIVisualEffectViewAdjuster {
    static var key = "visualEffectViewAdjuster"
    private weak var effectView: UIVisualEffectView!
    fileprivate init(effectView: UIVisualEffectView) {
        self.effectView = effectView
        self.effect = effectView.effect
    }

    public var contentView: UIView { get { return effectView.contentView } }

    public var effectAmount: Float = 1 {
        didSet {
            updateAmount()
        }
    }
    private var animator: UIViewPropertyAnimator? = nil

    public var effect: UIVisualEffect? {
        didSet {
            animator = resetAnimation()
        }
    }

    private func resetAnimation() -> UIViewPropertyAnimator {
        if let animator = animator {
            animator.stopAnimation(true)
            self.animator = nil
        }
        effectView.effect = effect
        effectView.contentView.subviews.forEach { $0.alpha = 1.0 }
        let animator_ = UIViewPropertyAnimator(duration: 20, curve: .linear) {
            self.effectView.effect = nil
            self.effectView.contentView.subviews.forEach { $0.alpha = 0.0 }
        }
        return animator_
    }

    private func updateAmount() {
        if(effectAmount == 0) {
            if(animator?.state != .stopped) {
                animator?.stopAnimation(true)
                animator = nil
            }
            effectView.effect = nil
        } else if(effectAmount == 1) {
            if(animator?.state != .stopped) {
                animator?.stopAnimation(true)
                animator = nil
            }
            effectView.effect = effect
        } else if(effect != nil) {
            animator = animator ?? resetAnimation()

            animator!.fractionComplete = CGFloat(min(max(1-effectAmount, 10e-8), 1-10e-8))
        }
    }
}