You have no good reason to believe the widespread lie that blue is for vegetative growth. People have been successfully growing plants under just HPS with almost 0% blue for over 40 years. The main benefits of added blue light are shorter stems and the development of pigments that protect plants from sunburn when transferred outdoors.

"have 30% added blue spectrum"

That's probably another big lie. A quick Google search shows "30% increase in the blue spectrum." You don't know the difference? 30% of 0 is 0. If you are growing seedlings for transplanting outdoors, don't use just HPS. If you intend to keep the plants under artificial light, HPS is adequate.

"250W 4200K MH - generic brand"

You should find a spectral distribution chart for the bulb. If the spectrum has a large spike near 590nm (caused by sodium), you should avoid it because there is ample evidence that this wavelength is bad for plants. See http://forums.gardenweb.com/discussions/3698430/fyi-evidence-that-yellow-light-585-595nm-is-bad-for-plants?n=1 .

Some MH bulbs do not have such a spike, but, they cost more. A color rendering index (CRI) > 90 guarantees that there is no large 590nm spike.

this is the MH bulb, it's actually 4000k not 4200 as i thought: http://www.eiko.com/Products.aspx?ProductIndex=MH250/U
couldn't find any chart for the bulb, it has a 70 CRI..

as for the HPS, i'm aware that you can grow under HPS only, but wanted to avoid the longer stretched stems. i will say the added 30% blue seems to be accurate because i do notice the HPS bulb fires up with very visible blue light. it's not a cheap china bulb, it's made in the US so i don't see why it would be a lie, since i see there are other well known brands who have engineered HPS bulb to provide more blue.

im starting the plants indoors and then they'll be going outdoors when the weather warms.

I have a Super Agro bulb in my HPS and everything I have put under it has flourished. I don't use it on veggies, though- just to overwinter orchids and citrus plants but all flower and produce really well under it.

Even those most demanding of sun flower well. If your plants get leggy, elevate them a bit closer to the unit and keep a fan blowing over them.

"I have a Super Agro"

Have you ever moved something grown under only that HPS outdoors into full sunlight, and, if so, what happened?

"couldn't find any chart for the bulb, it has a 70 CRI.."

It probably looks much like this:

http://www.eyehortilux.com/img/products/spectrals/mh.jpg

Notice the spike at 590 nm. That is what you want to avoid.

If you get a MH bulb, you should get one with a spectral chart similar to this:

http://www.eyehortilux.com/img/products/spectrals/blue.jpg

because it has no spike at 590 nm. Apparently, such MH bulbs are expensive. See

http://www.eyehortilux.com/products/metal-halide (click Hortilux Blue)

"Have you ever moved something grown under only that HPS outdoors into full sunlight, and, if so, what happened?"

I do it all the time- nothing "happens."

Even the super sensitive Meyer lemon never skips a beat or drops a leaf. This time of years things are out for a week, in for a few days, then back out again and I never have a problem.

I do occasionally start new plants or buy something during the indoor months- typically when I move them outdoors for the very first time I give them a day or two sheltered from high noon sun just to err on the side of caution. But I don't really coddle anything- generally once past frost everything is out for good with no adaptations whatsoever.

"I do it all the time- nothing "happens."

Well, that also answer's the Op's implied question about which bulb to use. ( I notice that the OP never used "?".)