I am working on upgrading a website from v9 to v10 and it seems to be working except for this error when accessing a page that uses a property based on "Umbraco.BlockList".

The content page displays this message and then the JSON of the block:

    This content is no longer supported in this context.

    You might want to remove this block, or contact your developer to take actions for making this block available again.

Any ideas what the error might be?

    Received an error from the server
An error occurred
Sequence contains more than one matching element

Exception Details
System.InvalidOperationException, System.Private.CoreLib, Version=6.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e: Sequence contains more than one matching element
Stacktrace
at System.Linq.ThrowHelper.ThrowMoreThanOneMatchException()
   at System.Linq.Enumerable.TryGetSingle[TSource](IEnumerable`1 source, Func`2 predicate, Boolean& found)
   at Umbraco.Cms.Core.PropertyEditors.PropertyEditorCollection.get_Item(String alias)
   at Umbraco.Cms.Core.Models.Mapping.ContentPropertyBasicMapper`1.Map(IProperty property, TDestination dest, MapperContext context)
   at Umbraco.Cms.Core.Models.Mapping.ContentPropertyDisplayMapper.Map(IProperty originalProp, ContentPropertyDisplay dest, MapperContext context)
   at Umbraco.Cms.Core.Mapping.UmbracoMapper.Map[TTarget](Object source, Type sourceType, MapperContext context)
   at Umbraco.Cms.Core.Mapping.UmbracoMapper.Map[TSource,TTarget](TSource source, MapperContext context)
   at System.Linq.Enumerable.SelectIPartitionIterator`2.MoveNext()
   at System.Linq.Enumerable.WhereEnumerableIterator`1.ToList()
   at System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source)
   at Umbraco.Cms.Core.Models.Mapping.TabsAndPropertiesMapper.MapProperties(IContentBase content, List`1 properties, MapperContext context)
   at Umbraco.Cms.Core.Models.Mapping.TabsAndPropertiesMapper`1.Map(TSource source, MapperContext context)
   at Umbraco.Cms.Web.BackOffice.Mapping.ContentMapDefinition.Map(IContent source, ContentVariantDisplay target, MapperContext context)
   at Umbraco.Cms.Core.Mapping.UmbracoMapper.Map[TTarget](Object source, Type sourceType, MapperContext context)
   at Umbraco.Cms.Core.Mapping.UmbracoMapper.Map[TTarget](Object source, MapperContext context)
   at Umbraco.Cms.Core.Models.Mapping.ContentVariantMapper.Map[TVariant](IContent source, MapperContext context)
   at Umbraco.Cms.Web.BackOffice.Mapping.ContentMapDefinition.Map[TVariant](IContent source, ContentItemDisplay`1 target, MapperContext context)
   at Umbraco.Cms.Core.Mapping.UmbracoMapper.Map[TTarget](Object source, Type sourceType, MapperContext context)
   at Umbraco.Cms.Core.Mapping.UmbracoMapper.Map[TTarget](Object source, MapperContext context)
   at Umbraco.Cms.Web.BackOffice.Controllers.ContentController.MapToDisplay(IContent content, Action`1 contextOptions)
   at Umbraco.Cms.Web.BackOffice.Controllers.ContentController.GetEmpties(IEnumerable`1 contentTypes, Int32 parentId)
   at Umbraco.Cms.Web.BackOffice.Controllers.ContentController.GetEmptyByKeysInternal(Guid[] contentTypeKeys, Int32 parentId)
   at lambda_method1924(Closure , Object , Object[] )
   at Microsoft.AspNetCore.Mvc.Infrastructure.ActionMethodExecutor.SyncObjectResultExecutor.Execute(IActionResultTypeMapper mapper, ObjectMethodExecutor executor, Object controller, Object[] arguments)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.InvokeActionMethodAsync()
   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.Next(State& next, Scope& scope, Object& state, Boolean& isCompleted)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.InvokeNextActionFilterAsync()
--- End of stack trace from previous location ---
   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.Rethrow(ActionExecutedContextSealed context)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.Next(State& next, Scope& scope, Object& state, Boolean& isCompleted)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.InvokeInnerFilterAsync()
--- End of stack trace from previous location ---
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.<InvokeNextExceptionFilterAsync>g__Awaited|26_0(ResourceInvoker invoker, Task lastTask, State next, Scope scope, Object state, Boolean isCompleted)