In Robert Louis Stevenson's "Treasure Island," the pirates exchanged death threats by passing notes marked with "The Black Spot." In the Congress, the same message is conveyed via "the blue slip."

A "blue slip" is a parlimamentary form letter through which the House of Representatives can kill, irrevocable, certain bills passed by the Senate. Next week, when Congress comes back from recess, that fate may await, S. 790, the waterways toll bill.

It is a frustrating prospect for Sen. Pete Domenici (R.N.M.), the sponsor of the waterways toll bill, which would require, for the first time, that barge lines pay a fee for using inland waterways built and maintained by the federal government.

Despite opposition from some of the Senate's most powerful Democrates, Domenici had successfully steered his bill through two committees and onto the floor. There, two weeks ago, he staved off potentially crippling amendments and won passage of the waterways toll by a healthy 71-to-20 vote.

When the senators had finished action on the bill, it was passed on to the myriad clerks and secretaries responsible for informing that House that the Senate had acted.

The Senate's legislative clerk compiled, with scissors and tape, a final version of the toll bill containing all the last-minute amendments adopted during the floor debate.

The resulting compilation of paper went to the bill clerk, who logged it in his huge ledger and passed it to the enrolling elerk, who logged it in his ledger and passed it to the public printer, who printed a final text and passed it back to the enrolling clerk, who passed it to the secretary of the Senate, who certified its official passage by the Senate and passed it back to the enrolling clerk, who walked over to the Housed chamber. In a brief ceremony unchanged since the birth of Congress, he formally presented "a message from the Senate to the other body."

Beneath all the ancient protocol, however, lay a constitutional provision that made the waterways toll bill a possible candidate for a "blue slip."

Article I, Section 7 of the Constitution--one of the few clauses in that document that divides responsibilities between the two houses of Congress--provides that "all bills for raising revenue shall originate in the House of Representatives."

That sentence, known to scholars as the "origination clause," was based on 18th century practive in the British parliament, where only the House of Commons--considered the more representative of the two houses--could originate tax levies.

Although it is not frequently invoked, the "origination clause" is dear the hearts of the members of the House. They jealously guard their perogatives and they have prepared a form letter - printed on a blue slip of paper - to inform the Senate whenever they jeel-their power of objection has been violated by a Senate-passed bill.

Such a letter makes a Senate-passed bill a legal nullity in the House. And the Senate has no recourse.

As soon as the waterway tolls bill was formally delivered to the House--about a week after it passed in the Senate--Rep. Al Ullman (D-Ore.), chairman of the House's tax-writing Ways and Means Committee, asked his chief of staff, John Martin, to determine whether the Senate was treading on the House's contitutionally guaranteed turf.

Accordingly, opponents and proponents of waterway tolls quickly engulfed Martin in legal briefs arguing whether Domenici's bill is a proper target for an oringination clause " blue slip."

Like bands, slang, and dress styles, constitutional clauses tend to go in and out of fashion over the generations. The origination clause was highly popular among constitutional lawyers in the last quarter of the 19th Century, and has been invoked rarely since.

Thus Martin found himself reading briefs about a series of century-old Supreme Court cases in which citizens had challenged various federal taxes on ground that they had originated in the Senate.

The decisive issue in those cases seemed to be whether the "revenue raising" aspect of a particular bill was the bill's main feature or an incidental effect of legislation primarily designed for other purposes.

In 1897, for example, when Congress enacted a Senate-originated bill that taxed bank notes, the Supreme Court upheld the tax because its main purpose was to regulate banking, not to raise money.

There is some question, though, whether all the legal arguments would carry much weight if the water way tolls bill came up for a "blue slip" in the House.

Under House rules, a floor vote would be required on the origination clause question. But House members rarely vote against measures that seem to protect their own prerogatives. If Ullman were to decide that the waterway tolls bill violates the origination clause, the House would be estremely likely to vote for a "blue Slip."

So, just two weeks after their triumph on the Senate floor, Domenici and his pet bill were in serious trouble.