We build with both Blazor and React regularly, and the honest answer to "which is better" is that it depends on factors that have little to do with the frameworks themselves.
If your team is already strong in C# and .NET, Blazor lets you share code, models and validation logic between frontend and backend, and keeps your hiring and onboarding within one language ecosystem. For internal tools, admin dashboards and applications where your backend is already ASP.NET Core, that consistency is a real advantage.
React's strength is its ecosystem and the sheer number of engineers who already know it well. If you're building a highly interactive, design-heavy consumer product, or you expect to hire frontend specialists from a broad talent pool, React's maturity and component ecosystem is hard to beat.
Performance differences between the two are rarely the deciding factor for typical business applications — both are fast enough when built well. The deciding factor is almost always team composition, existing codebase and how important frontend/backend code-sharing is to your situation.
Our advice: don't choose based on which framework is trending. Choose based on what your team can maintain confidently three years from now, because that's usually a bigger cost than the initial build.