LevelManager.cs

using System.Collections;
using UnityEngine;
using UnityEngine.SceneManagement;

public class LevelManager : MonoBehaviour
{
    [SerializeField]
    private string[] levelNames;

    private ScreenWipe screenWipe;
    private int nextLevelIndex;

    private void Awake()
    {
        DontDestroyOnLoad(gameObject);
        screenWipe = FindObjectOfType<ScreenWipe>();
    }

    void Update()
    {
        if (Input.GetButtonDown("Fire1"))
            StartCoroutine(LoadNextLevel());
    }

    private IEnumerator LoadNextLevel()
    {
        nextLevelIndex++;
        if (nextLevelIndex >= levelNames.Length)
            nextLevelIndex = 0;

        string nextLevelName = levelNames[nextLevelIndex];

        screenWipe.ToggleWipe(true);
        while (!screenWipe.isDone)
            yield return null;

        AsyncOperation operation = SceneManager.LoadSceneAsync(nextLevelName);
        while (!operation.isDone)
            yield return null;

        screenWipe.ToggleWipe(false);
    }
}

ScreenWipe.cs

using UnityEngine;
using UnityEngine.UI;

public class ScreenWipe : MonoBehaviour
{
    [SerializeField]
    [Range(0.1f, 3f)]
    private float wipeSpeed = 1f;

    private Image image;

    private enum WipeMode { NotBlocked, WipingToNotBlocked, Blocked, WipingToBlocked }

    private WipeMode wipeMode = WipeMode.NotBlocked;

    private float wipeProgress;

    public bool isDone { get; private set; }

    private void Awake()
    {
        image = GetComponentInChildren<Image>();
        DontDestroyOnLoad(gameObject);
    }

    public void ToggleWipe(bool blockScreen)
    {
        isDone = false;
        if (blockScreen)
            wipeMode = WipeMode.WipingToBlocked;
        else
            wipeMode = WipeMode.WipingToNotBlocked;
    }
    
    private void Update()
    {
        switch(wipeMode)
        {
            case WipeMode.WipingToBlocked:
                WipeToBlocked();
                break;
            case WipeMode.WipingToNotBlocked:
                WipeToNotBlocked();
                break;
        }
    }

    private void WipeToBlocked()
    {
        wipeProgress += Time.deltaTime * (1f / wipeSpeed);
        image.fillAmount = wipeProgress;
        if (wipeProgress >= 1f)
        {
            isDone = true;
            wipeMode = WipeMode.Blocked;
        }
    }

    private void WipeToNotBlocked()
    {
        wipeProgress -= Time.deltaTime * (1f / wipeSpeed);
        image.fillAmount = wipeProgress;
        if (wipeProgress <= 0)
        {
            isDone = true;
            wipeMode = WipeMode.NotBlocked;
        }
    }

    [ContextMenu("Block")]
    private void Block() { ToggleWipe(true); }
    [ContextMenu("Clear")]
    private void Clear() { ToggleWipe(false); }

}