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I don't think there is a definitive history on white appearing in these dogs. Some older texts do have mentions of dogs with white, white dogs, etc., but nothing definite.
One of the parti clubs has several old texts on their website.
I've found mention in a book from 1894 of off colored dogs in the ring. Here's a quote from it...
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"There are some other rough-haired toy terriers, which are, however, of little account, because they have never been bred to any particular type. Occasionally wee things very like what a miniature Skye terrier would be are seen; and, again, some smart little dogs with cut ears, evidently a cross between a Yorkshire terrier and some other variety of small dog, are not at all uncommon, and were quite numerous before the dog show era commenced. Since then the general public will not look at anything other than what is considered to be of blue blood. At one of the early London shows separate classes were provided for Scotch terriers under 71b. weight and white in colour, fawns with the same limit, and blues likewise, each of the three attracting a fair entry, most of which were, however, what we should now call "cross-bred" broken-haired toy terriers."
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...and this is from the breed standard for the Yorkshire Terrier from the same book.
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"Coat
The hair as long and straight as possible (not wavy), which should be glossy, like silk (not woolly); colour, a bright steel blue, extending from the back of the head to the root of the tail, and on no account intermingled the least with fawn, light, or dark hairs."
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here's the link...
The Terriers. A History And Description Of The Modern Dogs Of Great Britain And Ireland | by Rawdon B. Lee