Q:

Is the origin of the term "blackleg" racist?

A blackleg is defined as:

a person who continues working when fellow workers are on strike

When did this term originate? Does it's origin have racist connotations?

A:

Its etymology seems to be without racist connotations; at least according to the website for National Coal Mining Museum for England:

Blackleg
  Term for a worker who breaks
  a strike and continues working. The name comes from working miners trying to hide the fact that they had been working could be found out if their trousers were rolled up: they would have black legs. See scab, strike breaker.
  — The 1984-5 Miners Strike Resource

On the other hand, wordsmith.org says 

noun:
  1. One who works while other workers are on strike.
  2. A swindler, especially in games such as gambling.
  3. One of various diseases of plants or cattle.  
ETYMOLOGY:
  It’s unclear how the term came to be employed for a strikebreaker. Earliest documented use: 1722.

... so it's hard to be 100% sure!

A:

There are different assumptions about the origin of the term used to refer to strikers who cheat going to work.
According to the following, the meaning derives from the bird rook known for its rapacious appetite and its black legs: 

The expression blackleg originated from the bird rook. As we all know, this bird is black in colour and has got black legs. Rooks are very cunning and they know how to steal food. Needless to say, few people like them. Even today, the term rook is sometimes used to refer to a person who takes advantage of gullible individuals. Since rooks have black legs, cheats are also called blacklegs. As time went by, this expression began to be used to refer to workers who cheat by going to work when their fellow employers are on strike. (English Grammar)

Another assumption is that the expression originated among  coal mine strikers:

The term is said to have come from strikes in the coal mines. Those who were on strike had washed and brushed up after their last trip down the mine and therefore anyone covered in coal dust was a strike-breaker - a blackleg. The derogatory term scab is also used for such people. It is not a direct synonym of strike-breaker since a blackleg is specifically someone who works at a job while his colleagues are on strike. (Words, Words and Phrases)

A:

From yet another source (unfortunately unavailable):

'Blackleg' dates from the very early 1700s. Eric Partridge gives
  alternative uses and origins. By 1722 it is certainly being used as a
  description of a disease affecting the legs of sheep and cattle.
  Tempting though it is to suggest that the earliest organised wage
  workers, the wool combers, who were noted for trade union militancy,
  used the term there is no direct evidence that this was so.
Another version of its origins has it as a gaming term, dating from
  1771. According to this view, blacklegs were firstly "turf-swindlers", the name coming from a fashion amongst them for wearing a certain kind
  of black boot. Another, related possibility is that gamecocks, used in
  the then very popular `sport' of cock-fighting, were invariably the
  possessors of black legs.
Yet another version of its origin is supposed to be from the mining
  industry. The term was certainly used in miners' songs of the 1830's.
  (See A L Lloyd's "Come All Ye Bold Miners - ballads and songs of the
  coalfields" [1978] ", published by Lawrence and Wishart, for various
  examples.) This raises the question of whether it is a word special to
  the mining industry in origin. For this was the period when the word
  "blacksheep" was current. It has often been suggested that in the
  context of the coal industry the word `blacklegs' has a double edge to
  it. For, in the days before pithead baths, a working miner in a strike
  situation could easily be found by the simple expedient of lifting his
  trouser leg to discover his own leg blackened by coal dust! This seems
  a little fanciful, whilst there is no academic backing for the notion.
  After all, mining strikes took place in closed communities where there
  was little chance of discovering a wayward spirit. There could however
  be some derisory value involved here and the sporting origin -
  especially of cock fighting - would fit the social milieu of the
  collier better.
From this account it may be readily seen that no racist intent or
  connotation is involved in the term "blackleg", arising from the use
  of the word `black' as a negative force. Nonetheless, modern dislike
  of the term arises from the method whereby the word "black" is
  frequently used in this way- as in black arts or witchcraft, black
  mood, black day, black outlook, etc. etc.

Sidenote: In Swedish it is a similar word word for this "svartfot" (black foot), probably translated from the English word. 
Funnily enough(?) there are other racist sounding words for similar issues that aren't either. "Gulingar" (Yellows), comes from employer friendly unions that had was called The Yellow Union. And we also have "bruntungor" (brown tounges) which is a euphemism for "kiss ass" as you might imagine why.

