That looks odd, it should say "Other Block Types".

To set your own text you would normally use GroupName in your ContentTypeAttribute:

[ContentType(DisplayName = "Banner block",
        GUID = "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx", 
        GroupName = "My blocks")]
public class BannerBlock: BlockData {}

Thanks Erik!

Page and Block group names are best put in a static class as shown here https://world.episerver.com/documentation/developer-guides/CMS/Content/grouping-content-types-and-properties/

This allows you to control the ordering and also apply permissions so groups can only been seen by certain users (If you need it) it also keeps everything together rather than duplicate strings across multiple blocks/pages.

Example

/// <summary>
/// The page group names used to group pages
/// </summary>
[GroupDefinitions]
public static class PageBlockGroupNames
{
    [Display(Order = 1000)]
     public const string Home = "Home";

    [Display(Order = 2000)]
     public const string Search = "Search";
}

As you can see in my example we increment in thousands. This is a tip that allows you to easily insert a group inbetween later without reorganizing all your groups.