Scope, architecture, timeline, cost: the document that makes a project ship.
A blueprint is the plan you build from. Done well, it's the cheapest insurance you can buy against a stalled or runaway project. Here's what a good one actually contains, and how to tell a real plan from a sales document.
Scope: what's in, and what's deliberately out
A good blueprint states exactly what you're building and, just as importantly, what you're not. Vague scope is how projects balloon. Explicit out-of-scope items are a feature of the plan, not an omission.
Architecture: how it's put together
You don't need to understand every technical detail, but the blueprint should show the major pieces, how they connect, and why. Good architecture is what lets you add your next ten features without rebuilding from scratch.
Tech stack, with reasons
It should name the technologies and, briefly, why they were chosen for your situation, not just because they're trendy. The reasoning matters more than the names.
Timeline in phases
A credible timeline is broken into phases with visible milestones, not one big date at the end. Phases let you see progress, test early, and adjust without derailing everything.
Cost you can plan around
The blueprint should tie cost to scope, so you understand what you're paying for and what would change the number. Surprise costs almost always trace back to vague scope.
Risks and assumptions, stated openly
Every real plan has assumptions and risks. A good blueprint names them instead of hiding them, so there are far fewer nasty surprises later.
You should own it
A blueprint is yours. If you decide to build elsewhere, you should be able to take it with you. If a partner won't hand it over, that tells you something important.