Q:

Does grass absorb juglone from black walnut trees?

If grass grows near a black walnut tree, will they absorb the infamous juglone? If so, would it it be a lot or would plants near them be okay to eat/feed still?

A:

Don't know why I'm being told my answer was deleted, but I'll try adding another one since I was in the process of editing it when it was deleted.
I cut my neighbor's grass and its a pain to ride the mower under the low-hanging limbs of the black walnut trees to cut the thick, tall grass which grows robustly under them.  If juglone has any negative effects on grass, I am not witness to them.  The pines and oaks (to lesser extent) are a different story.
Regarding the comment:

I'm asking if plants absorb and retain the juglone. Basically, does it poison stuff that does grow in it?

Ohio State University Extension Fact Sheet

Many trees, vines, shrubs, groundcovers, annuals and perennials will grow in close proximity to a walnut tree. Certain cultivars of "resistant" species are reported to do poorly.

This seems to indicate that even if a plant is resistant, its still affected and must be taking it up to some degree. Although I have a fair understanding how plants exchange H+ ions for other cations in the soil solution, I don't have an understanding how plants take up complex compounds.... but I know they do.  The articles I have read on the matter, mostly repeat what I just said... they don't know either.
Interestingly enough, kentucky bluegrass appears to grow better under black walnut trees:

The beneficial effect of black walnut on pastures in encouraging the growth of Kentucky bluegrass (Poa pratensis L.) and other grasses appears to be valid as long as there is sufficient sunlight and water.

In any case, it seems likely that plants readily absorb juglone.. whether its toxic to them or beneficical.
Other quotes of interest may include:

Juglone is poorly soluble in water and does not move very far in the soil.
Walnut leaves can be composted because the toxin breaks down when exposed to air, water and bacteria. The toxic effect can be degraded in two to four weeks. In soil, breakdown may take up to two months. 
Black walnut leaves may be composted separately, and the finished compost tested for toxicity by planting tomato seedlings in it. Sawdust mulch, fresh sawdust or chips from street tree prunings from black walnut are not suggested for plants sensitive to juglone, such as blueberry or other plants that are sensitive to juglone. However, composting of bark for a minimum of six months provides a safe mulch even for plants sensitive to juglone.

It would be interesting to see a study where bluegrass grown under walnut trees and then used as mulch for tomato plants would inhibit the growth of the tomato plants. Unfortunately, I can find no such study.  My guess is yes, there would be some effects.
In light of this, I'm going to save the grass I cut from under my neighbor's trees and use it to fertilize my bluegrass lawn.  Hopefully the juglone in the grass will help my grass grow and hinder the growth of the weeds.  Perfect! :)

