For clarification: I'm not interested in answers using custom properties with composite controls here which I think could be used to solve this. 

Using a property of a block type on your model is the preferred way to acomplish this. Then you can add whatever properties that should be "grouped" to the block type.

Ah, I did not realize one could add block types directly as "page type property types".

I tried it out using:

    public class ImageBlock : EPiServer.Core.BlockData
    {
        [UIHint(UIHint.Image)]
        [Display(GroupName = SystemTabNames.Content)]
        public virtual Url ImageUrl { get; set; }

        [Display(GroupName = SystemTabNames.Content)]
        [CultureSpecific]
        public virtual string AlternateText { get; set; }
    }

And it works like a charm! :) Thanks!

Edit: In the page type class I used by block type like 

        [Display(
            GroupName = SystemTabNames.Content,
            Order = 360)]
        public virtual ImageBlock ImageBlock { get; set; }