I am searching for a groundcover or companion plants for some ornamental grasses, namely blue festuca and hakonechloa.
How about more grass? Moor Grass, to be specific, Blue Moor Grass, AKA Sesleria caerulea. It's compact, neat and hardy. The blades are blue on the top side, and dark green on the under side. They grow twisted, so both colors show at the same time. It's very easy to grow from seed, and will take a wide range of soil, sun, and, once established, water.
Donn is a true grass lover so it does not surprise me that he recommended more grasses :-)
OTOH, I like the look of different leaf textures and forms together, so I do combine my grasses with other plants to provide contrast. I have hakone grass in one area paired with variegated London Pride, Saxifraga x urbium 'Aureovariegata' - the saxifrage forms rosettes of evergreen foliage, spotted with yellow and tolerates shade well, even dry shade. It will produce airy stalks of pale pink flowers in late spring. I also have some Lamium 'Anne Greenaway' in another area with golden hakone - the gold colors compliment each other. Epimediums would also make a great combo, as the new growth is often tinged red, as are the stems of the hakone.
Blue fescue is most often grown in full sun and so requires different companions. One of my favorites would be sunroses, helianthemums, but those may not be hardy for you. Low growing sedums or stonecrops would work well and those with reddish tones would provide an attractive contrast. And a classic combo is fescue with woolly thyme - the foliage colors are nearly identical, they enjoy the same hot sunny position and the pink thyme flowers look great with the silvery blue of the fescue.