Presumably your startup is going to offer value to the "Email Marketers/Web Designs/Developers" you're targeting. You've thought about how to create value for them, and looked to make your offering and its presentation work for them.
But now you're not thinking about what would be valuable for the bloggers you've identified. You're just thinking, in effect,
- I need people to beta trial my web app
- You have an audience
- Tell them to try me out
Sure, you could strike it lucky. But more likely, you're causing annoyance. We all need to start the day reminding ourselves (courtesy of Chris Dixon), "no one cares about my stupid little start up."
So first off, you need to make your primary activity reaching out directly to your audience. Find out who's talking about the kind of problem you've set out to solve, and build conversations and relationships. Do that well and you'll be rewarded by committed triallists.
And secondly, you need to invest time in getting to know those 12 bloggers individually. How do they see the world? What's their stance in areas you know about? What are the services they promote, and why? What are the services they disparage, and why? What do you know about their readership - anything distinctive? Then you'll have a better idea about how to approach them in a way that is more than just ask.
Then, when you've got your trial group, you'll let them know about my new web app too, won't you?