I love Parahebes. There is a white one, a blue one and one with rosy pink flowers that I discovered recently. they are evergreen and flower for months. You can just trim the spent flower heads off and they will start flowering again. They have a light character which would maybe allow the iris to show through. Hebes and Sarcococca would perhaps be too heavy to allow the iris to shine through. You could also go for grasses, I like Carex Evereste for it's white variegation and it is at its best in winter. But with grasses, you might lose your Iris. There are some nice Thymes that might do the job, and also prostrate Rosemary. The blue flowers of the Rosemary would look super with the blue Iris and they flower at roughly the same time. Rosemary looks good all year around and the root is 'central' so you wouldn't lose your Iris to them. Hope you find a good solution :)

Prostrate Germander (Teucrium x lucidrys 'Prostratum')?

also think of Veronica prostrate, usually small blue speedwell flowers but there are other colours.
if its not in full sun [unlikely if you have iris] then there are things like Mitella and ajuga

Rock roses (Helianthemum) will do the job, but they do tend to sprawl all round for quite a way so will spread beyond the planting area. You can prune them back to keep them tidy, but they usually get at least 18 inches wide. Otherwise, Campanula portenschlagiana (previously C. muralis, sometimes still sold as that) is a neat, evergreen grower - but don't buy C. poscharskyana or carpatica instead of, or by mistake, they sprawl all over the shop, messy growers in comparison to C. muralis.

Both of these suggestions were found on this website https://www.craigiehallnursery.co.uk/ourshop/cat_1003096-All-view-all-alpines.html
How about Dianthus alpinus 'Joan's Blood'
Dark green mats, large, dark red flowers. A stunning form - large, red flowers with a dark central zone first appear in late spring over the mats of dark green, often bronzed foliage and continue off and on all summer.
Even the unopened buds look good - dark buds on dark stems over dark leaves doesn't sound good but it looks great. Small and special for a special place. Suitable for a trough. About 7cm (3") high.
Likes a fertile soil or compost but still well-drained and may benefit from the occasional liquid feed over summer - liquid tomato fertiliser is good but only at half rate.
Acaena microphylla 'Kupferteppich'
Kupferteppich (Copper Carpet) grows as spreading mats of finely cut leaves, coppery grey in colour. The flowers appear in summer as little white globes and develop into spiky red burrs which persist into autumn - these look especially good against the dark foliage.
Prefers a sunny site and is tolerant of most free-draining soils. 5cm (2") high. An easily grown plant ideal for covering larger areas, especially in or around paving and gravel or over walls. It can make good cover for dwarf bulbs too.