To be possibly pedantic, they never used the term dialectical materialism. That was Dietzgen. Later Plekhanov stitched together a philosophy of dialectical materialism out of Marx and Engels writings with reference to Hegel.
I don't see Marx and Engels as particularly unique as philosophers. Banality is a good quality for a philosopher and Marx and Engels could be quite banal. The dazzling philosophical obsessions of the leading intellectuals of the day are a distraction and satire is the appropriate response (Holy Family, German Ideology, Anti-Duhring). Good sense and a view to see the world as it is in all its nuance is a fine counterbalance.
Sorry Knotted, but you touched on a little pet hate there. At university, they would say Marx was a, for example, technological determinist because he said "quotes about windmills etc". For me, Marx didn't write Bibles, just to be recited. He gave a methodology of analysis. And in my opinion, to do him fair, we should apply the analysis to his works, and ask what would Marx say of himself if he was here today. We have to capture the essence of what he was talking about. [Dialectics; Essence?]
So yes, he may not have used the phrase dialectical materialism, but he certainly was. That Marx was a young Hegelian, a dialectician [. To coin a phrase] is without doubt. And this is where the point about him being banal comes in. Of course, he wasn't original, that is ONE of the points of dialectics, all we see is synthesis of precedents, synthesis of thesis and anti-thesis. Synthesis of the dialectics of Hegel and the materialism Newton etc, was Marx's achievement. Why an achievement?
That said most of Marx and Engels philosophical commentary is excellent. Marx's preface to The Contribution to the Critique of Political Economy is excellent. The point about the mode of presentation is central. Marx looks at political economy afresh, without making it seem like the unfolding of some law of nature. Capital is thus a very self-conscious and philosophical set of volumes that continually set capital as a product of history. I don't find the more explicitly philosophical language in Grundrisse useful, though. It makes it sound clunkingly mechanical. (Similarly the mathematical manuscripts are bloody awful).
I find it completely strange when people like yourself lord to political economy works etc, but then go on to ridicule Dialectics. What Marx produced, when he produced work such as capital, was like a map so we could navigate the landscape of the political economy. What do you need when you set out to draw a map? A compass. Dialectics was Marx and Engels compass, with which they could venture forth into the unmapped territory of the political economy.
I no longer feel the need for dialectical materialism. I can translate its terms into more precise language if I need to.
Didn't once Marx promise to produce a small pamphlet on the dialectic? Something which he never achieved? I would love to see your pamphlet that can translate the terms of dialectical materialism in a more precise language.