Yes you can cross: raspberries , blackberries , mulberries , wineberries ,dewberries , tayberries , boisenberries ,loganberries , salmonberries ,black caps, can all crossbreed .
There is even a "white blackberry" developed by Luther Burbank
The Nothernmost most cold resistant fruits in the world are berries. We should be using the most cold resistant fruits in the world to extend the range of all berries that they can crossbreed with .

See:

http://www.rosehybridizers.org/forum/message.php?topid=2986

for additional "possibilities".

Here is a link that might be useful: link for above

Wait a minute, farmfreedom! All of the brambles (blackberries, raspberries, etc.) are members of the rose family (Rosaceae), while mulberries are members of the Moraceae, along with figs and the rare succulent Dorstenia. There is virtually no structural similarity between the families: are you sure that mulberries and blackberries will hybridize?

Kevin : )

I believe if you look up the "TAY berry " or the "Logan berry " you will find that that is the cross breed. Also If this was not true How Did Luther Burbank develop the " White Blackberry " . He is also said to Have crossed Strawberries and Blackberries which I would like to see if anyone has a picture .
The Nothernmost most cold resistant fruits in the world are berries. We should be using the most cold resistant fruits in the world to extend the range of all berries that they can crossbreed with .

I'd like to see a strawberry/blackberry hybrid myself! At least they are in the same family.

Tayberries and loganberries are various hybrids of blackberries and raspberries, not either of those with mulberries. BTW, another member of the Moraceae that I forgot is the hedge apple, or Osage orange.

Kevin : )

I stand corrected Tayberries and loganberries and boysenberries are considered "dew berries" and they are related to rasberries. they belong to the genus "Rubus"

It appears the mulberries only looks like raspberries . the the mulberry family includes : breadfruit,hop,hemp (cannabis),fig and rubber plants, hedge apple, or Osage orange. and the paper mulberry (Broussonetia ).

Now what did Luther Burbank use to breed the "white blackberry"

is it possible that if you mixed a common edible fruit with one with bad side effects it may be poisonous?

Yes you can ! This was done with potatoes a while back to breed a pest resistant strain but they were poisonous to people.

hmm.... what about 2 edible plants? could they create a poison somehow?

I suppose if you crossed an almond (which is grown for its seed ) and an apricot (which is grown for its fruit ) and tried to eat the seed you might have a problem if you were sensitive and ate enough of them . Apricot pits are high in laetrile ( Prussic acid ) .

From a book I read--decades ago--about Luther Burbank, he used a yellow form of raspberry, and did multiple F2 and back crosses with blackberries to get the white color and fix it in a blackberry-type fruit. Reportedly, it tastes great, though I have never tried to grow it myself.

On the poisonous edible plant front, I hear that a company tried to breed celery for insect resistance, using normal selective breeding, not genetic modification. They succeeded, but in the process of multiplying it up, they discovered that many of the field workers were having poison-ivy-like reactions to it! "They dug, and they burned, and they dug, and they burned...." I never was in love with celery, but that's ridiculous.

Kevin : ])

Burbank's White Blackberry is truly white. I saw the plant with fruit at the Burbank house in Santa Rosa.
http://www.bulbnrose.org/Heredity/Burbank/WhiteBlackberry/WhiteBlackberry.html

The Raspberry x Strawberry hybrid was highly interesting, but sterile.
http://www.bulbnrose.org/Heredity/Burbank/Burbank_raspXstraw.html

Karl

Here is a link that might be useful: CybeRose & Bulbs