If you had EPiServer Find I would recommend using that to find them, if you have to use the built in API I would try with IContentLoader's GetDescendant() method.

var contentLoader = ServiceLocator.Current.GetInstance<IContentLoader>();
contentLoader.GetDescendents(new ContentReference(blockFolderId)).Select(pageReference => contentLoader.Get<BlockData>(pageReference)).Where(block => block is MyBlockType)

Note that depending on how many blocks you have this can be quite a heavy operation.

Hope this helps

Frederik  

You could use IContentModelUsage.ListContentOfContentType(ContentType contentType). 

Forgot to mention that the component that implements the interface can be retrieved from IOC container.

Hi Guys

Thank you very much for your response, On your comment Johan, is there a method to get the ContentType either by a Guid or Sytem.Type to pass to the ListContentOfContentType?

Many Thanks
Dave

There is a method Load on IContentTypeRepository that takes a type as parameter and returns a ContentType