My firm wants to use my personal blog as a platform to reach more people.

This is not something I want.

I don't know what to say, any tip to kindly answer such need?

Tell them No.
It's a hard thing to say but it needs to be a No.
Offer to setup a company one instead where they can follow your personal one. Perhaps even contribute to it.
Definitely a No.

Say no, there is no nicer answer.
“No I don’t feel it’s a good idea.”

Maybe they want to use your house to hire more people and your car to go to business trip, maybe later they will just stop paying you so they can hire someone else.

If they won't understand the distinction between a personal blog and a corporate one, then they likely won't make the distinction elsewhere, especially if you don't remind them of the significance of such distinctions.

As @dan-pud said, you can offer to help set up a corporate one instead, but I'd clarify what they want, first. If they genuinely want another platform to reach people from, more power to them. If they're being opportunistic and trying to subvert your blog for a quick exploitation of your existing audience, they are either being scummy or stupid and you shouldn't go anywhere near helping them.

Thanks for the feedback. I was in contact with them today and here is what they said. We would like to share your stuff with our colleagues.

I thought they wanted me to add their logo and such on my blog.

I guess if so, then this is fine.

@colaguy That does sound fine. Do keep an eye on things, just in case.

@powerfulparadox yeah, I will for sure! 🔥

Charge them? This kinds of things are not free.

@hack this. "You cant use it, but you can hire my services as a blogger for *insert current sallary here* and I'm sure you'll get great value out of everything on there!"

"Nope"