I helped to create the HubSpot Account Planning Template , which is a similar framework to the Blue Sheets. The problem with the template is that it sits outside of the HubSpot account, as a a stand alone document. As you have suggested, a deal or account plan should sit inside HubSpot ideally.
We work with a lot of HubSpot customers who sell to large customers and the two main methods of input for deal plans and account plans are the following:
Utilisting snippets (applicable to all users)
Using the team view (only available on the Sales Hub Enterprise plan).
We created a video which shows you the two methods and will hopefully help you in the short term