Similar to the page list in the alloy, i want to have a list of blocks with a certain category?
If you are retrieving list of blocks from page's content area, then one way to filter stuff out would be to cast to `ICategorizable`:
var items = currentPage.MainContentArea.FilteredItems
.Select(cai => _loader.Get<BlockData>(cai.ContentLink))
.Where(i => ((ICategorizable)i).Category.Contains(...));