Okay lavender or self blue breeds true. It is a diluted form of black but requires two of the lavender gene to show. If you breed your rooster to black hens(I would suggest all black hens for the easiest way to tell what you will get) you will get black birds that carry one copy of the lavender gene(aka black split to lavender).
Its a simple punnett square.
Lavender x lavender =lavender
Lavender x black= black lavender splits
Lavender x black/lav split = 50% black/lav splits and 50% lavenders
Black/lav splits can also be bred together which will give you approximately 25% lavender, 25% black/no split, 50% black/lavender splits.
Now the reason people told you to breed your lavender with blacks is because while lavender is a stunning color bred for generations the gene that causes the lavender color weakens and ruins feather quality. Periodically breeding back to blacks or keeping some blacks in your breeding pool helps keep feather quality.
If you want to to continue getting pure Orpingtons I would suggest finding some pure black Orpingtons and breeding them to your roo.
If that's impossible I would suggest the black Australorp.