Have you tried something along these lines?
var select = "SELECT HASHBYTES('MD5', ChunkData) FROM dbo.x where Id = @ID";
var parameter = new SqlParameter("ID", 40);
var query = context.Database.SqlQuery<byte[]>(select, parameter);
var buffer = query.FirstOrDefault();
If you are not required to use Entity Framework then a solution like this might be another option:
Getting binary data using SqlDataReader