It'll help , it all depends on how thick it is

There are many views about mulch, as I'm finding out. Compost works as a mulch on its own and many people say its the best mulch. Researchers at Iowa and Ohio State Universities found that 2 inches of compost prevents weed just as well as 2 inches of hardwood. Compost would add much more nutrients to the soil than the hardwood mulch, which actually reduces nitrogen at the soil level (just how much it reduces nitrogen is in debate, but the general consensus is that it would hurt perennials more than trees or shrubs).

Here is a link that might be useful: Ten Commandments of Mulch

The only advantage to adding a wood mulch over compost is to further suppress weed development. Compost tends to be a rather fertile seed bed and while a good layer of compost will suppress any underlying weed seeds, it simply provides a a very hospitable habitat for germinating windblown seeds. The good news is they are usually extremely easy to weed out as compost is generally quite loose and fluffy.

Otherwise, as newbie above (and who is learning fast!) points out, compost is just as efficient a mulch as bark or wood chips in moderating soil temperatures and conserving moisture and far more efficient in releasing nutrients. With any mulch, you need to apply 2-3 inches -- any less will not have the same effectiveness and more is just a waste.

As a wife, my gut says if the wife wants the black mulch, then get the black mulch! ;)

Seriously though, it's actually supposed to be a good thing to apply an inch or two of compost and then some wood mulch on top of that. The compost provides relatively quick nutrients, and the wood mulch helps to promote microbes in the soil. And especially if the area that you are mulching is shrubs, trees, or perennials, they tend to favor ammonium as a source of nitrogen instead of nitrates, which means that you needn't worry so much about the wood binding up the nitrogen for a while.

As long as you don't find out that there's something outright hazardous in the black mulch dye, I'd give it a try.

Any material you place on your soil to 1) aid in "weed" suppression, 2) aid in moisture retention, 3) aid in controlling soil temperature, and 4) aid in adding organic matter to your soil is mulch. I have no idea why some people distinguish between mulch, compost, and other stuff, or why people get so confused over what is a soil amendment and a mulch (the same material can be both).
Compost can be used as mulch and the sun beats down on the compost and dries it out which slows the bacteria tht are digesting it, some. Covering that compost with another mulch material, ie. wood chips, will help keep the compost more moist and the bacteria more active.
Does compost spread over the garden need to be covered with another material? It is your choice. Do you want to keep that compost moist and the bacteria more active or are you okay with letting the sun dry that compsot out?

I think it also depends on your zone. Here in the South, you have to have either hardwood or pine straw mulch. Otherwise even compost will dry out in the hot sun. I put compost around my plants but always have either pine bark or hardwood mulch over the bare areas. It helps keep the soil cool and retain moisture. Otherwise the topsoil, even if I have compost, will dry out and become as hard as cement.

Thanks, gardengal! This is a great place to learn.

The National Arboretum in D.C. uses compost without mulch in their camelia beds (so lots of trees about). I liked the look and that's what I tried for in my little streetside garden bed (about two feet wide between curb and privacy fence, very sunny). Looked pretty sharp, if you ask me. Then the townhouse association sent landscapers around to spread brown wood mulch over everything so there'd be a uniform look.

Why black mulch? It may overheat plants if you have hot summers.