Krysten, the best sugestion would be to touch base with the previous owner and duplicate whatever she tells you to do as best as you can.

Lack of blooms can be caused by weather that kills the flower buds, pruning after the flower buds have formed (they develop starting in August-September) and lack of moisture. There is little one can do about the weather so make sure that, if you have to prune, do so before August; and try to maintain the soil moist, not wet, not dry, as best as you can. Maintain 3-4" of acidic mulch up to the drip line of the shrub to help with moisture and cold/hot weather.

Trying to get a specific color or color hue out of your hydrangea is a totally different problem. The acidity or alkalinity in your soil will determine the color that you get. If your soil is naturally alkaline, the blooms will be a shade of pink. If you add aluminum sulphate, the soil pH will change and the blooms will develop a purplish color at first. If you continue increasing the acidity of the soil then the blooms will turn blue. Since they are already blue then it follows that you need to add lime so the soil will be less acidic and more alkaline. That eventually triggers a change from blue to purple and then then to pink. But be aware that this is very difficult with hydrangeas planted in the ground; potted hydrangeas are easier to deal with.

About the color purple..... the type of purple that you get as the plant changes from pink-to-purple-to-blue or from blue-purple-pink also depends on how many red pigments the plant actually has. The more it has, the more of a deep purple you get. What that means is that the type/variety of mophead hydrangea that you have will make a difference when you try to get a deep purple. Your plant may produce a light purple when shifting colors.

Varieties that make nice deep purples are Glowing Embers, Royal Purple (also called Mathilda Gutges I think), Merrit Supreme and others. The previous owner can tell you what types of purple she got and how easy/hard it was to achieve them.

Luis

Hi, Krysten
This is probably a older variety that blooms on old wood only. Based on its size and the fact that varieties that bloom on new and old wood (preferable in zone 5) are only a couple of years old.
Remember last April when we got SNOW just as the hydrangeas were optimistically budded? That probably killed off the blooms. Not every year is good for hydrangeas of this type in Michigan. Mine suffered.
Not much you can do about late snow and cold temps. It sounds as if it's too big to cover.
Here's what you can do. Definitely ask the owner. Don't pull back the mulch at the base until May 1. Don't prune until June 1. Before that, you can't tell what's dead.
Good luck!

Thanks everyone. I don't think I explained myself very well in my original post. I got TONS of blooms/flowers last year, but the actual flowers were greenish white. No color at all.

These Hydrangeas aren't mulched at all, I was thinking of trying the pine needle mulch stuff. I've heard that can change the PH enough to produce some color.

I'd contact the previous owner, but she was really standoffish with me about "her" gardens and acted a little put off by the fact that "kids" were buying her house (I'm 27, but might as well have been 9). She seemed pretty confident that I would kill all her landscaping.
Thanks again for the replies. I'll definitely look into your suggestions.

Many hydrangea macrophyllas (think: Endless Summers if you want an example) start blooming out in that similar way: green. Some have more/less color on the edges. But after a while, they turn blue/pink. If the bloom does not mature, the plant could be getting too much sun.

Feel free to add mulch. It protects the plant from temperature changes amd maintains moisture longer. However, I doubt mulch meaningfully changes soil pH. A little change, yes; a big change, no.

Krysten, it can be hard to leave a garden. The previous owner may have mellowed by now and even be flattered by a question from you indicating you appreciate her input, or even appreciate the work she did before you.

Any advice I were to give you about altering the PH wouldn't be from first hand continous experience...In my acidic soil I'd have to really dedicate myself to keeping the PH raised and even then I would still have blue overtones....good thing I like blue :)