demo.cs

using ProtoBuf;
using ProtoBuf.Serializers;
using System;
using System.IO;

static class P
{
    static void Main()
    {
        var obj = new SomeRegularPoco
        {
            Whatever = new BlobLike(new byte[] { 1, 2, 3 })
        };
        using var ms = new MemoryStream();
        
        Serializer.Serialize(ms, obj);
        Console.WriteLine(BitConverter.ToString(ms.ToArray()));
        // ^^^ 0A-03-01-02-03
        // i.e. "field 1, string; length 3; payload 1,2,3" - no extra layer
        ms.Position = 0;
        var clone = Serializer.Deserialize<SomeRegularPoco>(ms);
        Console.WriteLine(clone.Whatever.Payload.Length); // 3
    }
}


[ProtoContract]
public class SomeRegularPoco
{
    [ProtoMember(1)]
    public BlobLike Whatever { get; set; }
}

[ProtoContract(Serializer = typeof(BlobLikeSerializer), Name = "bytes")]
public readonly struct BlobLike
{
    public ReadOnlyMemory<byte> Payload { get; }
    public BlobLike(ReadOnlyMemory<byte> payload)
        => Payload = payload;

    public class BlobLikeSerializer : ISerializer<BlobLike>
    {
        SerializerFeatures ISerializer<BlobLike>.Features =>
                SerializerFeatures.CategoryScalar | SerializerFeatures.WireTypeString;

        BlobLike ISerializer<BlobLike>.Read(ref ProtoReader.State state, BlobLike value)
            => new BlobLike(state.AppendBytes(value.Payload));

        void ISerializer<BlobLike>.Write(ref ProtoWriter.State state, BlobLike value)
           => state.WriteBytes(value.Payload);
    }
}