可以将String.Split与两个串联的Environment.NewLine一起使用：

string[][] result = text.Split(new [] { Environment.NewLine + Environment.NewLine }, StringSplitOptions.None)
    .Select(token => token.Split(new []{Environment.NewLine}, StringSplitOptions.None))
    .ToArray();

https://dotnetfiddle.net/NinneE

按新行拆分输入是AOC的经典之作。下面是我的扩展Method.net7的一部分：

public static class Extention{
  /// <summary>
  /// Splits a text into lines.
  /// </summary>
  public static IEnumerable<string> Lines(this string text) => text.Split(Environment.NewLine, StringSplitOptions.RemoveEmptyEntries);

  /// <summary>
  /// Splits a text into blocks of lines. Split occurs on each empty line.
  /// </summary>
  public static IEnumerable<string> Blocks(this string text) => text.Trim().Split(Environment.NewLine + Environment.NewLine, StringSplitOptions.RemoveEmptyEntries);
}

代码为：

var result = text.Blocks()
                 .Select(b => b.Lines());

注：
.Split(Environment.NewLine + Environment.NewLine,为.NET 7