Where there is smoke there is fire. All the heart trouble that has happened to elite bodybuilders is something to note. Just taking hormone replacement can lead to a high red cell count and thick viscosity of blood. It's not to hard to extrapolate that using crazy bodybuilding doses of anabolic steroids could lead to heart attack at 55. It also increases blood pressure.
There is a difference between what you died from and what caused it. Don Youngblood died of a heart attack. That's a pretty vague description of a cause of death. Could have been an occlusion or it could have been V-Fib or many other things that lead to his hearts ability too keep pumping. If it was arterial build up one would have a pretty hard time blaming steroids as a very large percentage of the population has this and starts at a very young age. This is usually related to diet and of course genetics but I am of the opinion the genetics loads the gun and diet pull the trigger.
Blood getting "thick" is not up there as far as being a major cause for heart attack and I doubt that was the case with Don. I am not sure if an autopsy was done but I'm willing to bet it was plaque build up that caused his death. Just because someone exercises doesn't mean they are healthy and it doesn't mean there're arteries are clear. I know quite a few people who have had heart attacks in their 30's who were seemingly very healthy. None of them were drug users of any kind. Certainly steroids are not needed to lead a healthy life and may cause some health damage down the road depending on how much they are abused.
