Hellebores are not heavy feeders and I never fertilize mine. I have some huge clumps that are several feet across and bloom profusely. I do have excellent soil and mulch regularly with organic matter, typically with mushroom compost for the hellebores, as it tends to be less acidic.

If your soil is not ideal and fertilizing is necessary, a balanced organic or slow release (Osmacote) fertilizer applied in late summer will be of benefit, but it is a little late in the season now - all my caulescent hellebores have already set buds and these ferts tend to be temperature related anyway.

I'm of the opinion that you cannot overfeed a Hellebore. However you can feed them at the wrong time, so give them a generous feed of a balanced liquid fertiliser just after the flowers are going over (and through the summer if you want), this is when they're putting all their new growth for the season, root leaf and flower bud formation. I don't feed when in flower, or too late in the season when they can't use it.

Cheers, Greenmanplants