Mixing Async and Sync in same HTTP request ASP.NET Core C#
Well, generally it is not a problem to call a sync method within async method.

You are actualy doing this in more then few instances (GetExtension, NewGuild, ToString, etc...)

But if you have an async method available it would be shame to not use it.
Which you have - IFormFile.CopyToAsync().

using (var bits = new FileStream(path, FileMode.Create))
{
  await file.CopyToAsync(bits);
}
await _imageCropBlackBroderService.CropBlackBroderOfAnImageAsync(sourceFolderPath, destinationFolderPath);