See http://spec.graphql.org/October2021/#sec-Descriptions :

To allow GraphQL service designers to easily publish documentation alongside the
capabilities of a GraphQL service, GraphQL descriptions are defined using the
Markdown syntax (as specified by CommonMark). In the
type system definition language, these description strings (often {BlockString})
occur immediately before the definition they describe.

{BlockString} is used like a reference to some term but I don't see its definition. I can find only single usage of BlockString through the spec. I see BlockStringCharacter term and BlockStringValue function. I suppose that {BlockString} term here is used by mistake and should be changed to just block string.