Writing on an unidentified scroll of blank paper identifies paper

The character doesn't know that an unlabeled scroll is blank until
they look at the inside of the scroll, but that could be done
either by reading or by writing.

src/write.c

@@ -105,7 +105,7 @@ dowrite(struct obj *pen)
                  : "scroll";
     if (Blind) {
         if (!paper->dknown) {
-            You("don't know if that %s is blank or not.", typeword);
+            You("don't know whether that %s is blank or not.", typeword);
             return ECMD_OK;
         } else if (paper->oclass == SPBOOK_CLASS) {
             /* can't write a magic book while blind */
@@ -120,6 +120,7 @@ dowrite(struct obj *pen)
         exercise(A_WIS, FALSE);
         return ECMD_TIME;
     }
+    makeknown(SCR_BLANK_PAPER);
 
     /* what to write */
     Sprintf(qbuf, "What type of %s do you want to write?", typeword);