Use a virtual role to only show blocks available for the current site

SiteVirtualRole.cs

    public class SiteVirtualRole : VirtualRoleProviderBase
    {
        public string Site { get; set; }
        public override void Initialize(string name, NameValueCollection config)
        {
            base.Initialize(name, config);
            Site = config["site"];
        }
        public override bool IsInVirtualRole(IPrincipal principal, object context)
        {
            var site = HttpContext.Current?.Request?.Url?.Host;
            if (!string.IsNullOrEmpty(site) && (site.ToLower() == Site.ToLower())) return true;
            return false;
        }
    }

This is great but context is always null for me. Did you experience that?