Not sure how much voltage you need or the duration, but have you thought about using a typical gas barbeque ignitor?  No battery required.  Push the spring-loaded button and a spark is produced at the other end.

You may also find what you need at an RV supply store.  Many LP gas appliances use either manual or automatic pilot ignitors.  The same may be true for most natural gas appliances used in homes.

If you did not want to do what kpuddarby stated and want to pursue the other option, the black box would basically be wired as follows:

Use a switch that has a spring loading, i.e. when you push on the switch, path is closed but when you let off switch, it will return to normal open position.

Take the 12 volt positive terminal and wire to + side of coil, ground terminal should be ran to one side of switch. The other terminal of switch should be ran to - side of coil. Push switch momentarily (may have to hold for a second or two to charge coil) and releasing will fire coil via coil wire.

Hope this helps.
Tom

Sounds like what you need is a coil for an old T-Model Ford, which ran off of the battery (6 volts back then, I guess) and had a built-in set of make-break contacts, like a vibrator, plus (I think) a built-in condenser, that would give you a continuous spark.  As I recall, the spark was in the 1000-to-2000-volt range.  It has been about 40 years since I fooled around with one, but probably you can still buy brand-new T-Model Ford coils through antique car supply houses.

Have you thought about a proprietary hand-held HE igniter?  Chentronics in Norwich, NY make a neat hand-held battery operated unit called the Motivator - see their web-site: http://www.chentronics.com/Motivator.html

I am sure that any electronics store (Caddy Shack in the US, I'm in Australia)that sells kits would be able to give you an oscillator to suit your prupose. A good kit to try would be a Jacobs ladder kit, which produces a high voltage arc between two wires in a vertical V, the arc climbs the V getting longer as it climbs. You could have some fun with it before you convert it for your needs.

A system similar to this is used on mobile gas heaters. What you are looking is called either an ignition unit or vibrator. I think Janitrol used a coil on some of their heaters, but along with South Wind (Stewart-Warner Corporation) and other OEM's they make/made several types of ignition units. Most use a ignitor to light the fuel.

http://www.stewart-warner.com/combustion.html
http://www.kellyaerospace.com/janitrol.html

One solution to make a ocillating blackbox is to use a
NE-555 and build it up as a bistable switch. this unit will tolerate a surge of 1amp at 12Vdc and you only have to get two capasitors and two resistors in addition to the IC.
(If you use a precision potentiometers instead of the resistors you can adjust both hold time and cyclic duration
eg. how long the output is triggered and the total length of the wave)

You may find some info by searching on google.com

Link it up with a relay and use a standard coil.
(dont forget to protect the relay contacts with a capacitor)
(the back surge from a coil can become quite heavy).
the high woltage parts you may use from a car or other existing ignited engine.

If you don't find the info send me a note.
I have a little folder on the NE-555 IC somwhere in my garage.