import com.atlassian.jira.component.ComponentAccessor
import com.atlassian.jira.issue.CustomFieldManager
import com.atlassian.jira.issue.Issue
import com.atlassian.jira.issue.fields.CustomField
import com.atlassian.jira.issue.link.IssueLink

IssueLink issueLink = issue.get("issuelinks").first() { IssueLink il ->
    //if you're looking for the first issue that the current issue "blocks".
    // Note that if you wanted the opposite (is blocked by), you'd need to replace "Outward" by "Inward" here and "getDestinationObject" by "getSourceObject" below
    return il.getIssueLinkType().getOutward().equals("blocks");
}
if (issueLink==null)
    return null
Issue linkedIssue = issueLink.getDestinationObject()

final CustomFieldManager customFieldManager = ComponentAccessor.getCustomFieldManager()
final CustomField customField = customFieldManager.getCustomFieldObject("customfield_11505")

return linkedIssue.getCustomFieldValue(customField)