For blocklist elements that need a 'controller' I've daisy chained an AspNetCore ViewComponent (using Microsoft.AspNetCore.Mvc;) from my block template before. It needs its own model and its own view, but it works pretty well with async data sources such as an api or database call. The alternative would perhaps be to put a bunch of code in your template itself but that seems pretty bad from an MVC viewpoint. Implementing some caching of this data to avoid a zillion DB or API calls helps keep things performant.

https://learn.microsoft.com/en-us/aspnet/core/mvc/views/view-components?view=aspnetcore-8.0

Here is an example:

The template for the block list element ends up almost empty -- containing just an 'Invoke' command that takes us into the ViewComponent world.

@inherits Umbraco.Cms.Web.Common.Views.UmbracoViewPage<BlockGridItem>


@* HTML Markup is stored in Views/Components since this element is implemented as a component so it can have its own 'controller'-esque code behind *@
@*Parameters must be wrapped in an anonymous type with properties that match the invoke parameter names*@
@await Component.InvokeAsync(typeof(MyCustomComponent), new { 
        myBlockElement = (MyBlockElement)Model.Content!
})

Then you need a "controller" for your ViewComponent that contains the Invoke method. This controller isn't a typical controller. It derives from ViewComponent but it can take DI injection args through its constructor just like a controller can -- such as a logger or your configuration object which might have an api endpoint or a connection string. Its invoke method can take whatever args you want but I have found it useful to take an instance of your block element. the naming of the args must match what is called by the view template above. It is temperamental anonymous type.

The view component also has its own template and its own model but I usually pass along my block element in the model

public class MyCustomComponent : ViewComponent
{
    // The invoke method is called when the component is first displayed
    // This invoke method can take args, passed when invoking from the parent view. 
    public async Task<IViewComponentResult> InvokeAsync(MyBlockElement myBlockElement)
    {

        var model = new MyCustomComponentViewModel();
        model.MyBlockElement = myBlockElement;
        try
        {
            model.MyExtraData = await GoGetSomeExternalData();
        }catch(Exception ex)
        {
            this.logger.LogError(ex, "Problem retrieving data for block grid element ");
        }

        return View(this.GetType().Name, model);
    }
}

The model is something that contains your block element PLUS whatever extra data you retrieve

 public class MyCustomComponentViewModel
 {
     // This can be whatever we want it to be. It can pass along fields from a block list element's content,
     // It might contain additional 'hijacking-added' properties along with the block list element

     public MyBlockElement MyBlockElement { get; set; } = null!;
     public ExtraData? MyExtraData { get; set; } = null;

 }

Then you create a subfolder in your Views directory for these ViewComponent views

-Views
    -MyCustomComponent
        -MyCustomComponent.cshtml

Where the cshtml file looks something like this

@model MyComponentViewModel
<div>@Model.MyBlockElement.NormalStuff</div>
<div>@Model.MyExtraData.Name</div>